Issue #2326891 by tim.plunkett, jhodgdon: Convert system_element_info() to Element classes.

core/includes/ajax.inc

core/includes/common.inc

core/includes/form.inc

core/includes/theme.inc

...ib/Drupal/Core/Field/Plugin/Field/FieldType/EmailItem.php

core/lib/Drupal/Core/Render/Element/Actions.php

core/lib/Drupal/Core/Render/Element/Ajax.php

core/lib/Drupal/Core/Render/Element/Button.php

core/lib/Drupal/Core/Render/Element/Checkbox.php

core/lib/Drupal/Core/Render/Element/Checkboxes.php

core/lib/Drupal/Core/Render/Element/Color.php

.../Drupal/Core/Render/Element/CompositeFormElementTrait.php

core/lib/Drupal/Core/Render/Element/Container.php

core/lib/Drupal/Core/Render/Element/Date.php

core/lib/Drupal/Core/Render/Element/Details.php

core/lib/Drupal/Core/Render/Element/Dropbutton.php

core/lib/Drupal/Core/Render/Element/ElementInterface.php

core/lib/Drupal/Core/Render/Element/Email.php

core/lib/Drupal/Core/Render/Element/Fieldgroup.php

core/lib/Drupal/Core/Render/Element/Fieldset.php