[NbtkScrollBar] fix page scrolling on horizontal scrollbars
[NbtkScrollBar] fix page scrolling on horizontal scrollbars

Horizontal scrollbars would only page-scroll in one direction.

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