[DX] Form API: Allow individual options in select elements to be set to disabled

This was inspired by the discussion in https://backdrop.zulipchat.com/#narrow/stream/218635-Backdrop/topic/.24f...

Consider this form element:

  $form['my_checkboxes'] = array(
    '#type' => 'checkboxes',
    '#title' => t('my Checkboxes'),
    '#options' => array(
      1 => 'one',
      2 => 'two',
      3 => 'three',
      4 => 'four',
    ),
    '#description' => t('This is the description for the entire set of checkboxes'),
    '#default_value' => array(1, 3),
  );

One can set descriptions for each individual checkbox, and also disable only one/some of these checkboxes:

  $form['my_checkboxes'][1]['#description'] = t('This is the description for the 1st checkbox');
  $form['my_checkboxes'][3]['#description'] = t('This is the description for the 3rd checkbox');
  $form['my_checkboxes'][2]['#disabled'] = TRUE;

The above would result in this:

---

Although individual descriptions do not make sense for select elements, there are perfectly valid cases where you need to: - make sure that one/some of the options are disabled/locked (user cannot select them, but they are still shown) - make sure that one/some of the options are selected + at the same time disabled/locked (so that you can force one or more options that must be selected in addition to those that the user can select).

However, if you consider this select element:

  $form['my_select'] = array(
    '#type' => 'select',
    '#title' => t('my Select'),
    '#options' => array(
      1 => 'one',
      2 => 'two',
      3 => 'three',
      4 => 'four',
    ),
    '#default_value' => array(1, 3),
  );

...or this multi-select element:

  $form['my_select'] = array(
    '#type' => 'select',
    '#multiple' => TRUE,
    '#title' => t('my multi-Select'),
    '#options' => array(
      'group1' => array(
        1 => 'one',
        2 => 'two',
      ),
      'group2' => array(
        3 => 'three',
        4 => 'four',
      ),
    ),
    '#default_value' => array(1, 3),
  );

Then this has absolutely no effect:

$form['my_select'][2]['#disabled'] = TRUE;