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

tests/interactive/scroll-view-sizing.js

@@ -338,7 +338,7 @@ function toggleFade(button) {
 	button.label = 'No';
 	break;
     }
-    scrollView.set_vfade(vfade.label == 'Yes');
+    scrollView.set_style_class_name(button.label == 'Yes' ? 'vfade' : '');
 }
 
 vfade.connect('clicked', function() { toggleFade(vfade); });