Make #base_type a standard way to extend #type in renderable arrays.

Problem/Motivation

Currently there is no way to define a new #type by extending an existing #type using hook_element_info(). Every #type is totally "flat" and there's a lot of duplicated configuration between types.

Consider the following example (there are many in core like this):

  // Input elements.
  $types['submit'] = array(
    '#input' => TRUE,
    '#name' => 'op',
    '#is_button' => TRUE,
    '#executes_submit_callback' => TRUE,
    '#limit_validation_errors' => FALSE,
    '#process' => array('form_process_button', 'ajax_process_form'),
    '#pre_render' => array('form_pre_render_button'),
    '#theme_wrappers' => array('input__submit'),
  );
  $types['button'] = array(
    '#input' => TRUE,
    '#name' => 'op',
    '#is_button' => TRUE,
    '#executes_submit_callback' => FALSE,
    '#limit_validation_errors' => FALSE,
    '#process' => array('form_process_button', 'ajax_process_form'),
    '#pre_render' => array('form_pre_render_button'),
    '#theme_wrappers' => array('input__button'),
  );
  $types['image_button'] = array(
    '#input' => TRUE,
    '#is_button' => TRUE,
    '#executes_submit_callback' => TRUE,
    '#limit_validation_errors' => FALSE,
    '#process' => array('form_process_button', 'ajax_process_form'),
    '#return_value' => TRUE,
    '#has_garbage_value' => TRUE,
    '#src' => NULL,
    '#pre_render' => array('form_pre_render_image_button'),
    '#theme_wrappers' => array('input__image_button'),
  );

Would it not be simpler and easier to understand and maintain to do the following:

  // Input elements.
  $types['submit'] = array(
    '#base_type' => 'button',
    '#executes_submit_callback' => TRUE,
    '#theme_wrappers' => array('input__submit'),
  );
  $types['button'] = array(
    '#input' => TRUE,
    '#name' => 'op',
    '#is_button' => TRUE,
    '#executes_submit_callback' => FALSE,
    '#limit_validation_errors' => FALSE,
    '#process' => array('form_process_button', 'ajax_process_form'),
    '#pre_render' => array('form_pre_render_button'),
    '#theme_wrappers' => array('input__button'),
  );
  $types['image_button'] = array(
    '#base_type' => 'button',
    '#executes_submit_callback' => TRUE,
    '#name' => NULL,
    '#return_value' => TRUE,
    '#has_garbage_value' => TRUE,
    '#src' => NULL,
    '#pre_render' => array('form_pre_render_image_button'),
    '#theme_wrappers' => array('input__image_button'),
  );

This is actually how the 'text_format' FAPI #type currently works with the "non standard" #base_type attribute - https://api.drupal.org/api/drupal/developer%21topics%21forms_api_referen...

Proposed resolution

Allow \Drupal\Core\Render\ElementInfo to build #type information recursively.

Remaining tasks

Write code.

User interface changes

None.

API changes

An element #type definition can extend from another #type using the #base_type attribute