IGraphicElement interface clean-up and optimizations.
Animating a GraphicElement that doesn't share the Group's DO should be now faster and smoother since redrawing it won't redraw the Group anymore.
1. Group doesn't always clear the first sequence of display objects now
2. Moved the shared DO logic almost entirely into Group
3. More granular invalidation for GraphicElements
QE Notes: Make sure we have test that count the number of display objects for a given set of graphic elements and a group
Reviewer: Glenn, Ryan, Jason