Change our menu building blocks to use PHP templates so that themes

can override them and define their own menu formats.  I worry a little
bit that this approach may be too heavy since we're now doing a lot
more template includes than we were before.  Also, I had to change the
Menu API to stop using __toString() because you can't throw exceptions
from __toString() which would make it an unhappy experience for
developers.

modules/gallery/libraries/Admin_View.php

@@ -47,8 +47,7 @@ class Admin_View_Core extends Gallery_View {
   public function admin_menu() {
     $menu = Menu::factory("root");
     module::event("admin_menu", $menu, $this);
-    $menu->compact();
-    return $menu;
+    return $menu->compact()->render();
   }
 
   /**