shell-global: add a "display" property

and update callers to fetch that rather than doing
"global.screen.get_display()"

https://bugzilla.gnome.org/show_bug.cgi?id=654639

js/ui/panel.js

@@ -237,7 +237,6 @@ AppMenuButton.prototype = {
 
     _init: function() {
         PanelMenu.Button.prototype._init.call(this, 0.0);
-        this._metaDisplay = global.screen.get_display();
         this._startingApps = [];
 
         this._targetApp = null;