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/messageTray.js

@@ -19,6 +19,7 @@ const SUMMARY_TIMEOUT = 1;
 const HIDE_TIMEOUT = 0.2;
 
 const ICON_SIZE = 24;
+const BUTTON_ICON_SIZE = 36;
 
 const MAX_SOURCE_TITLE_WIDTH = 180;
 
@@ -262,8 +263,16 @@ Notification.prototype = {
             this._buttonBox = box;
         }
 
-        let button = new St.Button({ style_class: 'notification-button',
-                                     label: label });
+        let button = new St.Button();
+
+        if (Gtk.IconTheme.get_default().has_icon(id)) {
+            button.add_style_class_name('notification-icon-button');
+            button.child = St.TextureCache.get_default().load_icon_name(id, BUTTON_ICON_SIZE);
+        } else {
+            button.add_style_class_name('notification-button');
+            button.label = label;
+        }
+
         this._buttonBox.add(button);
         button.connect('clicked', Lang.bind(this, function() { this.emit('action-invoked', id); }));
     },

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'
         ];
     },