overviewControls: Move reactive flag to main actor

The main overview actor was made reactive to catch scroll-events
and propagate them; after some code shuffling, the actor that
catches scroll events ended up not being the same actor that's
supposed to propagate this, which broke using the scroll wheel
to switch workspaces.

https://bugzilla.gnome.org/show_bug.cgi?id=700595

js/ui/overviewControls.js

@@ -506,10 +506,10 @@ const ControlsManager = new Lang.Class({
         this.indicatorActor = this._indicator.actor;
 
         this.actor = new St.Widget({ layout_manager: new Clutter.BinLayout(),
+                                     reactive: true,
                                      x_expand: true, y_expand: true,
                                      clip_to_allocation: true });
         this._group = new St.BoxLayout({ name: 'overview-group',
-                                        reactive: true,
                                         x_expand: true, y_expand: true });
         this.actor.add_actor(this._group);