[D8][DX] Provide a generic 'entity_autocomplete' Form API element

This is the same feature as https://www.drupal.org/node/1959806 which was added in Drupal core 8.0.x, and it would be a great DX+ for custom and contrib modules. We also have at least one UX use case in core that would take advantage of that feature: #1822, and we also have a todo item for this in EntityLayoutContext::form(), in https://github.com/backdrop/backdrop/blob/1.x/core/modules/layout/plugin...

...
    $form['id'] = array(
      '#title' => t('Enter the ID of a @entity', array('@entity' => $info['label'])),
      '#type' => 'number',
      '#default_value' => $input,
      '#step' => 1,
      '#min' => 1,
      // @todo: Backdrop doesnt yet have a comprehensive entity autocomplete
      // function. We'd need to write a custom autocomplete function that
      // handles all entity types. This would be best as a core function
      // rather than just for this code.
      // '#autocomplete_path' => 'node/autocomplete',
      '#weight' => -10,
    );
...

Description:

The autocomplete field widgets provided by the Entity reference module have been refactored into a generic 'entity_autocomplete' Form API element which can be used in regular form structures (i.e. not only in entity forms).

Usage examples:

Provide a basic autocomplete element that matches node titles from all bundles:

$form['my_element'] = array(
  '#type' => 'entity_autocomplete',
  '#target_type' => 'node',
  // #default_value can be either an entity object or an array of entity objects.
  '#default_value' => $entity,
);

If we want to restrict the matches to a single or a set of bundles, we can use the 'target_bundles' selection setting:

$form['my_element'] = array(
  '#type' => 'entity_autocomplete',
  '#target_type' => 'node',
  '#selection_handler' => 'default', // Optional. The default selection handler is pre-populated to 'default'.
  '#selection_settings' => array(
    'target_bundles' => array('article', 'page'),
  ),
);

For more usage examples, see the change record: New 'entity_autocomplete' form element added