StScrollView: Implement real fade effect

Implement an edge fade effect (top/bottom) using a
ClutterOffscreenEffect subclass, replacing the former
shadow hack.

https://bugzilla.gnome.org/show_bug.cgi?id=639460

js/ui/searchDisplay.js

@@ -166,7 +166,7 @@ SearchResults.prototype = {
 
         let scrollView = new St.ScrollView({ x_fill: true,
                                              y_fill: false,
-                                             vshadows: true });
+                                             vfade: true });
         scrollView.set_policy(Gtk.PolicyType.NEVER, Gtk.PolicyType.AUTOMATIC);
         scrollView.add_actor(this._content);