Add icon buttons to notifications

Now if an id is passed to Notification.addButton that matches a theme icon,
it will be displayed as an icon.  This is another in the implement the
music player mockups series.
http://live.gnome.org/GnomeShell/Design/Guidelines/MessageTray/MusicPlayer

https://bugzilla.gnome.org/show_bug.cgi?id=621014

js/ui/notificationDaemon.js

@@ -245,8 +245,9 @@ NotificationDaemon.prototype = {
             // 'body-images',
             'body-markup',
             // 'icon-multi',
-            'icon-static'
+            'icon-static',
             // 'sound',
+            'x-gnome-icon-buttons'
         ];
     },