- Modified UBvgSubsetAdaptor to correctly save and load strokes, so the
transform matrices that were saved are now loaded correctly.

- Added handling of mousePress / Move / Release events to
UBGraphicsStrokesGroup, so that the transform matrix is calculated and
stored after moving a pen stroke directly (by clicking on it, and not on
its frame). Note: this duplicates quite a bit of code that is in
UBGraphicsDelegateFrame. It may be best to go back and modify both
classes so that the same functions can be called when moving a stroke.