Refactoring of 'admin_toolbar_search.module'...

https://www.drupal.org/project/admin_toolbar/issues/3551183

Refactored files admin_toolbar_search.css and admin_toolbar_search.module files:

• Updated Functional tests accordingly.
• Renamed admin_toolbar_search CSS file.
• Added comments in module's yaml files.
• Fixed Keyboard shortcut displaying in field's title or placeholder if it is disabled.
• Replaced search toolbar tab link with <nolink> instead of the admin index URL.
• Refactored all module's HTML ID strings intro a constants class AdminToolbarSearchConstants.

Moved related CSS changes to MR !177 (merged).