st-scroll-view: Make the fade effect and offset themable

Theme authors now have the power (and responsibility) of creating fade
effects with the new CSS length property '-st-fade-offset'. A value of
0 disables the effect.

This new CSS approach replaces the current programmatic toggle of
the 'vfade' property. A new CSS style class name 'vfade' is used as
a replacement for the old property.

https://bugzilla.gnome.org/show_bug.cgi?id=651813

js/ui/searchDisplay.js

@@ -192,7 +192,7 @@ SearchResults.prototype = {
 
         let scrollView = new St.ScrollView({ x_fill: true,
                                              y_fill: false,
-                                             vfade: true });
+                                             style_class: 'vfade' });
         scrollView.set_policy(Gtk.PolicyType.NEVER, Gtk.PolicyType.AUTOMATIC);
         scrollView.add_actor(this._content);