[scroll-bar] Use adjustment_interpolate_relative
[scroll-bar] Use adjustment_interpolate_relative

Instead of setting the value, use mx_adjustment_interpolate_relative()
so you get smooth scrolling, like with MxScrollView.

jonnylamb/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx