ScreenShield: decouple detailed notifications from resident notifications

The designs says that only music notifications should be shown in full
in the screenshield, the others should be either shown as a summary or
with very light details.

https://bugzilla.gnome.org/show_bug.cgi?id=685926

js/ui/notificationDaemon.js

@@ -501,6 +501,9 @@ const NotificationDaemon = new Lang.Class({
                 }));
         }
 
+        // Mark music notifications so they can be shown in the screen shield
+        notification.isMusic = (ndata.hints['category'] == 'x-gnome.music');
+
         let gicon = this._iconForNotificationData(icon, hints);
         let gimage = this._imageForNotificationData(hints);