Chats should jump to the top of the notification queue.

This is to ensure users get notified as soon as chats are received.
Notifications with critical urgency still have the highest priority.

https://bugzilla.gnome.org/show_bug.cgi?id=630934

js/ui/telepathyClient.js

@@ -476,6 +476,7 @@ Source.prototype = {
         }
 
         this._notification = new Notification(this);
+        this._notification.setUrgency(MessageTray.Urgency.HIGH);
 
         // Since we only create sources when receiving a message, this
         // is a plausible default