Rewrite the layout code of the message tray

Previous code had a mixture of fixed positioning and ClutterBinLayout,
and this was broken badly for autorun notifications.
Rewrite to use ClutterBinLayout and Clutter properties exclusively.

https://bugzilla.gnome.org/show_bug.cgi?id=683378

js/ui/layout.js

@@ -120,7 +120,8 @@ const LayoutManager = new Lang.Class({
         this.panelBox.connect('allocation-changed',
                               Lang.bind(this, this._updatePanelBarriers));
 
-        this.trayBox = new St.BoxLayout({ name: 'trayBox' }); 
+        this.trayBox = new St.Widget({ name: 'trayBox',
+                                       layout_manager: new Clutter.BinLayout() }); 
         this.addChrome(this.trayBox);
         this.trayBox.connect('allocation-changed',
                              Lang.bind(this, this._updateTrayBarrier));