overview: Move overview actions and scrolling to background group

Both the Overview::scroll-event and actions added via addAction()
are meant to work anywhere in the overview, but for now only work
on the primary monitor. Move the handling to the background group
that is known to span all outputs to fix.

https://bugzilla.gnome.org/show_bug.cgi?id=766883

js/ui/overviewControls.js

@@ -421,7 +421,6 @@ const ControlsManager = new Lang.Class({
 
         let layout = new ControlsLayout();
         this.actor = new St.Widget({ layout_manager: layout,
-                                     reactive: true,
                                      x_expand: true, y_expand: true,
                                      clip_to_allocation: true });
         this._group = new St.BoxLayout({ name: 'overview-group',