messageTray: Make Tweener shut up about normal properties

Tweener needs an initial value to tween from, otherwise it won't
work, so give it something it can use.

js/ui/messageTray.js

@@ -1902,6 +1902,7 @@ const MessageTray = new Lang.Class({
 
         this.actor.add_actor(this._banner.actor);
 
+        this.actor._opacity = 0;
         this.actor.opacity = 0;
         this.actor.y = -this._banner.actor.height;
         this.actor.show();