overview: Add cover pane directly to overviewGroup

The cover pane is used to block events during transitions, but as
workspaces don't share the same container as other overview elements,
they are currently excempt from the event blocking.
Move the cover pane to the top-level overview container instead.

https://bugzilla.gnome.org/show_bug.cgi?id=709034

js/ui/overview.js

@@ -148,7 +148,7 @@ const Overview = new Lang.Class({
         // Dash elements, or mouseover handlers in the workspaces.
         this._coverPane = new Clutter.Actor({ opacity: 0,
                                               reactive: true });
-        this._stack.add_actor(this._coverPane);
+        Main.layoutManager.overviewGroup.add_child(this._coverPane);
         this._coverPane.connect('event', Lang.bind(this, function (actor, event) { return true; }));
 
         this._stack.add_actor(this._overview);