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.

modules/gallery/libraries/Theme_View.php

@@ -111,15 +111,15 @@ class Theme_View_Core extends Gallery_View {
     return $menu->compact();
   }
 
-  public function context_menu($item) {
+  public function context_menu($item, $thumbnail_css_selector) {
     $menu = Menu::factory("root")
       ->append(Menu::factory("submenu")
                ->id("context_menu")
                ->label(t("Options")))
       ->css_class("gContextMenu");
 
-    gallery::context_menu($menu, $this, $item);
-    module::event("context_menu", $menu, $this, $item);
+    gallery::context_menu($menu, $this, $item, $thumbnail_css_selector);
+    module::event("context_menu", $menu, $this, $item, $thumbnail_css_selector);
     return $menu->compact();
   }