Issue #3224178 by TR, vikashsoni, longwave, joachim, andypost: Remove theme...

 * implementing hook_theme(), which specifies the name of the hook, the input

 * "variables" used to provide data and options, and other information. Modules

 * implementing hook_theme() also need to provide a default implementation for

 * each of their theme hooks, normally in a Twig file, and they may also provide

 * each of their theme hooks in a Twig template file, and they may also provide

 * preprocessing functions. For example, the core Search module defines a theme

 * hook for a search result item in search_theme():

 * @code

 * rendered by the Twig template; the processed variables that the Twig template

 * receives are documented in the header of the default Twig template file.

 *

 * hook_theme() implementations can also specify that a theme hook

 * implementation is a theme function, but that is uncommon and not recommended.

 * Note that while Twig templates will auto-escape variables, theme functions

 * must explicitly escape any variables by using theme_render_and_autoescape().

 * Failure to do so is likely to result in security vulnerabilities. Theme

 * functions are deprecated in Drupal 8.0.x and will be removed before

 * Drupal 9.0.x. Use Twig templates instead.

 *

 * @section sec_overriding_theme_hooks Overriding Theme Hooks

 * Themes may register new theme hooks within a hook_theme() implementation, but

 * it is more common for themes to override default implementations provided by

 * templates directory. A good starting point for doing this is normally to

 * copy the default implementation template, and then modifying it as desired.

 *

 * In the uncommon case that a theme hook uses a theme function instead of a

 * template file, a module would provide a default implementation function

 * called theme_HOOK, where HOOK is the name of the theme hook (for example,

 * theme_search_result() would be the name of the function for search result

 * theming). In this case, a theme can override the default implementation by

 * defining a function called THEME_HOOK() in its THEME.theme file, where THEME

 * is the machine name of the theme (for example, 'bartik' is the machine name

 * of the core Bartik theme, and it would define a function called

 * bartik_search_result() in the bartik.theme file, if the search_result hook

 * implementation was a function instead of a template). Normally, copying the

 * default function is again a good starting point for overriding its behavior.

 * Again, note that theme functions (unlike templates) must explicitly escape

 * variables using theme_render_and_autoescape() or risk security

 * vulnerabilities. Theme functions are deprecated in Drupal 8.0.x and will be

 * removed before Drupal 9.0.x. Use Twig templates instead.

 *

 * @section sec_preprocess_templates Preprocessing for Template Files

 * If the theme implementation is a template file, several functions are called

 * before the template file is invoked to modify the variables that are passed

 * - THEME_preprocess_HOOK(&$variables): Allows the theme to set necessary

 *   variables specific to the particular theme hook.

 *

 * @section sec_preprocess_functions Preprocessing for Theme Functions

 * If the theming implementation is a function, only the theme-hook-specific

 * preprocess functions (the ones ending in _HOOK) are called from the list

 * above. This is because theme hooks with function implementations need to be

 * fast, and calling the non-theme-hook-specific preprocess functions for them

 * would incur a noticeable performance penalty.

 *

 * @section sec_suggestions Theme hook suggestions

 * In some cases, instead of calling the base theme hook implementation (either

 * the default provided by the module that defined the hook, or the override

 *   hook_theme() implementation specifies variable 'foo', then in a render

 *   array, you would provide this data using property '#foo'. Modules

 *   implementing hook_theme() also need to provide a default implementation for

 *   each of their theme hooks, normally in a Twig file. For more information

 *   each of their theme hooks in a Twig template file. For more information

 *   and to discover available theme hooks, see the documentation of

 *   hook_theme() and the

 *   @link themeable Default theme implementations topic. @endlink

 * Preprocess theme variables for templates.

 *

 * This hook allows modules to preprocess theme variables for theme templates.

 * It is called for all theme hooks implemented as templates, but not for theme

 * hooks implemented as functions. hook_preprocess_HOOK() can be used to

 * preprocess variables for a specific theme hook, whether implemented as a

 * template or function.

 * It is called for all theme hooks. hook_preprocess_HOOK() can be used to

 * preprocess variables for a specific theme hook.

 *

 * For more detailed information, see the

 * @link themeable Theme system overview topic @endlink.

    $hooks = theme_get_registry();

  }

  // Determine the primary theme function argument.

  // Determine the primary theme hook argument.

  if (isset($hooks[$hook]['variables'])) {

    $keys = array_keys($hooks[$hook]['variables']);

    $key = $keys[0];

/**

 * Provides alternate named suggestions for a specific theme hook.

 *

 * This hook allows modules to provide alternative theme function or template

 * name suggestions.

 * This hook allows modules to provide alternative template name suggestions.

 *

 * HOOK is the least-specific version of the hook being called. For example, if

 * '#theme' => 'node__article' is called, then hook_theme_suggestions_node()

 * @endcode

 *

 * @param array $suggestions

 *   An array of alternate, more specific names for template files or theme

 *   functions.

 *   An array of alternate, more specific names for template files.

 * @param array $variables

 *   An array of variables passed to the theme hook. Note that this hook is

 *   invoked before any variable preprocessing.

/**

 * Alters named suggestions for a specific theme hook.

 *

 * This hook allows any module or theme to provide alternative theme function or

 * template name suggestions and reorder or remove suggestions provided by

 * This hook allows any module or theme to provide alternative template name

 * suggestions and reorder or remove suggestions provided by

 * hook_theme_suggestions_HOOK() or by earlier invocations of this hook.

 *

 * HOOK is the least-specific version of the hook being called. For example, if

 *     where the array keys are the names of the variables, and the array

 *     values are the default values if they are not given in the render array.

 *     Template implementations receive each array key as a variable in the

 *     template file (so they must be legal PHP/Twig variable names). Function

 *     implementations are passed the variables in a single $variables function

 *     argument. If you are using these variables in a render array, prefix the

 *     variable names defined here with a #.

 *     template file (so they must be legal PHP/Twig variable names).

 *   - render element: Used for render element items only: the name of the

 *     renderable element or element tree to pass to the theme function. This

 *     renderable element or element tree to pass to the template. This

 *     name is used as the name of the variable that holds the renderable

 *     element or tree in preprocess and process functions.

 *   - file: The file the implementation resides in. This file will be included

 *     prior to the theme being rendered, to make sure that the function or

 *     preprocess function (as needed) is actually loaded.

 *     prior to the theme being rendered, to make sure that the preprocess

 *     functions in this file are actually loaded.

 *   - path: Override the path of the file to be used. Ordinarily the module or

 *     theme path will be used, but if the file will not be in the default

 *     path, include it here. This path should be relative to the Drupal root

 *     directory.

 *   - template: If specified, the theme implementation is a template file, and

 *     this is the template name. Do not add 'html.twig' on the end of the

 *     template name. The extension will be added automatically by the default

 *     rendering engine (which is Twig.) If 'path' is specified, 'template'

 *     should also be specified. If neither 'template' nor 'function' are

 *   - template: If specified, this is the template name. Do not add 'html.twig'

 *     on the end of the template name. The extension will be added

 *     automatically by the default rendering engine (which is Twig.) If 'path'

 *     is specified, 'template' should also be specified. If 'template' is not

 *     specified, a default template name will be assumed. For example, if a

 *     module registers the 'search_result' theme hook, 'search-result' will be

 *     assigned as its template name.

 *   - function: (deprecated in Drupal 8.0.x, will be removed in Drupal 9.0.x)

 *     If specified, this will be the function name to invoke for this

 *     implementation. If neither 'template' nor 'function' are specified, a

 *     default template name will be assumed. See above for more details.

 *   - base hook: Used for theme suggestions only: the base theme hook name.

 *     Instead of this suggestion's implementation being used directly, the base

 *     hook will be invoked with this implementation as its first suggestion.

 *     HOOK is the base hook) changes the suggestion order, a different

 *     suggestion may be used in place of this suggestion. If after

 *     hook_theme_suggestions_HOOK() this suggestion remains the first

 *     suggestion, then this suggestion's function or template will be used to

 *     generate the rendered output.

 *     suggestion, then this suggestion's template will be used to generate the

 *     rendered output.

 *   - pattern: A regular expression pattern to be used to allow this theme

 *     implementation to have a dynamic name. The convention is to use __ to

 *     differentiate the dynamic portion of the theme. For example, to allow