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/workspace.js

@@ -161,7 +161,7 @@ WindowClone.prototype = {
             // will look funny.
 
             if (!this._selected &&
-                this.metaWindow != global.screen.get_display().focus_window)
+                this.metaWindow != global.display.focus_window)
                 this._zoomEnd();
         }
     },