Refactored: bootstrap dropdown theme hook

While the bootstrap_btn_dropdown theme hook was "technically" converted, it never really went under any real scrutiny. Nor does it really allow the many different features defined in Bootstrap documentation:
http://getbootstrap.com/components/#dropdowns
http://getbootstrap.com/components/#btn-dropdowns
http://getbootstrap.com/javascript/#dropdowns

The following issues were addressed and improved:

• Renamed bootstrap_btn_dropdown to bootstrap_dropdown. This theme hook name was inaccurate and misleading. Not all dropdown toggles are a button.
• Label processing.
• Attribute/class processing.
• Removed legacy and unused bootstrap_links theme hook for item list processing
• Added dropdown alignment options.
• Added support for detecting headers and divider items.

If you happen to have code that uses these old functions, please refactor them to use the bootstrap_dropdown theme hook instead:

  // This is an example of how to create complex dropdowns using render arrays
  // as items. See how to create simple links in the last example at the bottom.
  $items = array();

  // Create headers using #markup and #header.
  $items[] = array(
    '#markup' => _bootstrap_icon('envelope') . ' ' . t('Notifications'),
    '#header' => TRUE,
  );
  $items[] = array(
    '#type' => 'link',
    '#title' => t('Messages'),
    '#href' => 'messages',
    '#options' => array(
      'html' => TRUE,
    ),
  );

  // Created dividers by passing NULL or and empty string ('') as the item.
  $items[] = NULL;

  $items[] = array(
    '#markup' => _bootstrap_icon('user') . ' ' . t('My Account'),
    '#header' => TRUE,
  );
  $items[] = array(
    '#type' => 'link',
    '#title' => t('View profile'),
    '#href' => 'user',
    '#options' => array(
      'html' => TRUE,
    ),
  );
  $items[] = array(
    '#type' => 'link',
    '#title' => t('Logout'),
    '#href' => 'user/logout',
  );

  // Example of using bootstrap_dropdown in render arrays. Because render arrays
  // typically defer rendering until the last possible second, this allows the
  // array to be altered if needed. This is the preferred method.
  $dropdown = array(
    '#theme' => 'bootstrap_dropdown',

    // By default if you pass text (string) as the toggle, it will be converted
    // to an empty link (href="#"). You can, however, pass a render array
    // instead if a different element is needed (like a button).
    '#toggle' => array(
      '#type' => 'button',
      '#value' => format_username($GLOBALS['user']) . ' ' . _bootstrap_icon('chevron-down'),
    ),

    // Use the above complex items array.
    '#items' => $items,

    // Default styling causes dropdowns to position below and on the left edge
    // of the toggle. If you need to change the alignment you can pass either
    // of the following as alignments: "right" or "up".
    '#alignment' => array('right'),
  );
  print drupal_render($dropdown);



  // This second example shows how to use it with theme(). Note: because this
  // theme hook renders as an element, you need to pass all variables (prefixed
  // with a hash (#)) inside an element variable. This method is not preferred.
  print theme('bootstrap_dropdown', array(
    'element' => array(
      '#toggle' => t('Links') . ' ' . _bootstrap_icon('chevron-down'),
      // Despite how this dropdown is rendered, passing simple links as items
      // would work for both rendering methods. Below you can see a very simple
      // array of links.
      '#items' => array(
        l(t('Drupal.org'), 'http://drupal.org'),
        l(t('Drupal Groups'), 'http://groups.drupal.org'),
        l(t('Drupal API'), 'http://api.drupal.org'),
      ),
    ),
  ));