overview: Move ::scroll-event signal handling (again)

Commit c39ffa111 moved the signal handling from the controls- to the
background-group to enable scrolling on non-primary monitors.
However this broke scrolling on reactive overview elements as the
workspace switcher, as they're not descendants of the background.
To fix, move scroll-event handling to the overview group itself,
which is the common ancestor of all overview elements.

https://bugzilla.gnome.org/show_bug.cgi?id=768316

js/ui/layout.js

@@ -220,7 +220,8 @@ const LayoutManager = new Lang.Class({
         global.stage.add_child(this.uiGroup);
 
         this.overviewGroup = new St.Widget({ name: 'overviewGroup',
-                                             visible: false });
+                                             visible: false,
+                                             reactive: true });
         this.addChrome(this.overviewGroup);
 
         this.screenShieldGroup = new St.Widget({ name: 'screenShieldGroup',