Create the concept of an "ajax link" and use it for thumbnail rotation

and album covers in the context menu.

Notes:
- This requires context_menu() to have a CSS selector that refers to the
  <img> that we're operating on, otherwise we don't know how to find the
  thumbnail, etc.
- Create Menu_Element_Ajax_Link which has an ajax_handler attribute
  that contains a snippet of JS that we're going to run when the ajax
  call returns.
- Add $.gallery_replace_image in gallery.common.js
- Add lib/gallery.ajax.js which can be used to ajaxify any link, and have
  ui.init.js in the themes call that on all .gAjaxLink elements.

themes/admin_default/views/admin.html.php

@@ -25,6 +25,7 @@
    <script type="text/javascript">
    var MSG_CANCEL = "<?= t('Cancel') ?>";
    </script>
+   <?= $theme->script("gallery.ajax.js") ?>
    <?= $theme->script("gallery.dialog.js") ?>
    <?= $theme->script("superfish/js/superfish.js") ?>
    <?= $theme->script("jquery.dropshadow.js") ?>