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