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/overview.js

@@ -124,9 +124,6 @@ const Overview = new Lang.Class({
         Main.layoutManager.overviewGroup.add_child(this._backgroundGroup);
         this._bgManagers = [];
 
-        this._backgroundGroup.connect('scroll-event',
-                                      Lang.bind(this, this._onScrollEvent));
-
         this._desktopFade = new St.Widget();
         Main.layoutManager.overviewGroup.add_child(this._desktopFade);
 
@@ -155,6 +152,9 @@ const Overview = new Lang.Class({
             dragMotion: Lang.bind(this, this._onDragMotion)
         };
 
+
+        Main.layoutManager.overviewGroup.connect('scroll-event',
+                                                 Lang.bind(this, this._onScrollEvent));
         Main.xdndHandler.connect('drag-begin', Lang.bind(this, this._onDragBegin));
         Main.xdndHandler.connect('drag-end', Lang.bind(this, this._onDragEnd));