[NbtkScrollBar] add a vertical mode
[NbtkScrollBar] add a vertical mode

Add a vertical mode to scrollbar, so that it can represent a vertical
scroll bar without being rotated.

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