Drupal: Mimic menu_attributes behavior to add a "CSS classes" textfield to menu item form.

gistfile1.php

<?php

/**
 * Implements hook_form_FORM_ID_alter().
 *
 * Adds menu 'class' attribute options to the edit menu item form.
 */
function mymodule_form_menu_edit_item_alter(&$form, $form_state) {
  // Get item
  $item = $form['original_item']['#value'];

  // Get class default value
  if (isset($item['options']['attributes']) && isset($item['options']['attributes']['class'])) {
    $default_value = $item['options']['attributes']['class'];
  }
  else {$default_value = '';}

  // Change 'options' type
  $form['options'] = array(
    '#type' => 'item',
    '#tree' => TRUE,
    '#weight' => 50
  );
  
  // Create attributes item
  $form['options']['attributes'] = array('#type' => 'item', '#tree' => TRUE);

  // Add class textfield & item
  $form['options']['attributes']['class'] = array(
    '#type' => 'textfield',
    '#title' => t("CSS classes"),
    '#default_value' => $default_value,
    '#description' => t("Enter CSS classes, separated by spaces.")
  );
}