Allow `title_display` override for parts of `datelist` form element

@dipakmdhrm I've updated the fork/MR with a slight change so that it uses the value that's set in the form array rather than a fixed 'invisible'

I am mildly confused why this can't be preprocessed back in normally?

/**
 * Implements hook_preprocess_HOOK() for form_element__datelist.
 */
function my_theme_preprocess_form_element__datelist(array &$variables) {
  foreach (Element::children($variables['element']) as $child) {
    $variables['element'][$child]['#title_display'] = 'invisible';
  }
}

should work

The separate elements are added in a process callback, so the preprocess hook would be too early won't it?