[NbtkTooltip] don't allow the tooltip to be larger than the stage
[NbtkTooltip] don't allow the tooltip to be larger than the stage

This prevents the tooltip from going off screen by forcing the tooltip to
ellipsize.

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