[NbtkButton] allow buttons with both labels and icons
[NbtkButton] allow buttons with both labels and icons

Implement icons beside text in NbtkButton using an internal table

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