messageTray: Glue the notification to the bottom of the screen, always

We'll animate notifications popping up with another system soon enough,
instead. The idea here is that instead of carefully animating the Y
position of the notificationWidget when a notification updates, we
simply animate the height of the new actor inside the notification.
This will fix some of the awkward updates where instead of the
notification content expanding, we see the buttons or action area
pushed off the edge of the screen...

Animations that happen as a result of adding something new to the
notification or expanding it should be done by tweeing the new actors
in inside the notification.

js/ui/components/telepathyClient.js

@@ -737,8 +737,6 @@ const ChatNotification = new Lang.Class({
         this._lastMessageBox.add(body, props.childProps);
         this.addActor(this._lastMessageBox);
 
-        this.updated();
-
         let timestamp = props.timestamp;
         this._history.unshift({ actor: body, time: timestamp,
                                 realMessage: group != 'meta' });