Couple of changes to Group and DataGroup:
1. Got rid of content_changing, content_changed, dataProvider_changing, and dataProvider_changed events. The events both group and datagroup emit are the item add and item remove events. Fixed up Selector, DataComponent, and ItemsComponent to not key off of these events anymore.
2. Clean up Group's canShareDisplayObject from the blendMode change
3. Fix up DataGroup to deal with a case where a GraphicElement's itemRenderer's group was holding on to the display object. When the graphic element was pushed into another itemRenderer, we tried to remove the graphic element's display object from its parent, but calling removeChild() on a Group RTE's. We need to revisit the fix here, and possibly add methods for removing display objects to Group, or don't RTE in Group on removeChild/addChild, or add some code so itemRenderer's can clean themselves up afterwards...at any rate, this fix will be revisited.