[NbtkTooltip] prevent placing the tooltip horizontally off screen
[NbtkTooltip] prevent placing the tooltip horizontally off screen

This prevents the tooltip from being off screen either left or right. It
also re-positions the arrow as appropriate.

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