[NbtkWidget] Switch to having seperate background-image and border-image
[NbtkWidget] Switch to having seperate background-image and border-image

The "border-image" css property is now implemented as an extra centered
overlay image on the widget. It is painted on top of the border-image but
behind the child.

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