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/Menu.php

@@ -96,6 +96,37 @@ class Menu_Element_Link extends Menu_Element {
   }
 }
 
+/**
+ * Menu element that provides an AJAX link.
+ */
+class Menu_Element_Ajax_Link extends Menu_Element {
+  public $ajax_handler;
+
+  /**
+   * Set the AJAX handler
+   * @chainable
+   */
+  public function ajax_handler($ajax_handler) {
+    $this->ajax_handler = $ajax_handler;
+    return $this;
+  }
+
+  public function __toString() {
+    if (isset($this->css_id) && !empty($this->css_id)) {
+      $css_id = " id=\"$this->css_id\"";
+    } else {
+      $css_id = "";
+    }
+    if (isset($this->css_class) && !empty($this->css_class)) {
+      $css_class = " $this->css_class";
+    } else {
+      $css_class = "";
+    }
+    return "<li><a$css_id class=\"gAjaxLink $css_class\" href=\"$this->url\" " .
+      "title=\"$this->label\" ajax_handler=\"$this->ajax_handler\">$this->label</a></li>";
+  }
+}
+
 /**
  * Menu element that provides a pop-up dialog
  */
@@ -132,6 +163,9 @@ class Menu_Core extends Menu_Element {
     case "link":
       return new Menu_Element_Link($type);
 
+    case "ajax_link":
+      return new Menu_Element_Ajax_Link($type);
+
     case "dialog":
       return new Menu_Element_Dialog($type);