layout: add panelBox and trayBox

Have LayoutManager automatically deal with sizing and positioning
boxes for the panel and messageTray relative to the monitors.

Also, now that LayoutManager knows exactly where and how tall the
panel and tray are, have it manage the pointer barriers as well.

https://bugzilla.gnome.org/show_bug.cgi?id=612662

js/ui/main.js

@@ -192,8 +192,6 @@ function start() {
     ExtensionSystem.init();
     ExtensionSystem.loadExtensions();
 
-    panel.startupAnimation();
-
     global.display.connect('overlay-key', Lang.bind(overview, overview.toggle));
 
     global.stage.connect('captured-event', _globalKeyPressHandler);