[NbtkTable] fix issue with row span and row spacing
[NbtkTable] fix issue with row span and row spacing

Row spacing was being taken into account twice

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