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/appDisplay.js

@@ -40,7 +40,7 @@ AlphabeticalView.prototype = {
         this.actor = new St.ScrollView({ x_fill: true,
                                          y_fill: false,
                                          y_align: St.Align.START,
-                                         vshadows: true });
+                                         vfade: true });
         this.actor.add_actor(box);
         this.actor.set_policy(Gtk.PolicyType.NEVER, Gtk.PolicyType.AUTOMATIC);
     },