Optimize Group layout invalidation.
This change adds two Boolean flags to track if Group?\226?\128?\153s layout is invalidated. This way whenever a GraphicElement?\226?\128?\153s invalidateDisplayList() is called, the Group won?\226?\128?\153t redo the layout, but only validate that particular GraphicElement?\226?\128?\153s display list. Similar for size. Also I removed unconditional updateDisplayList() while assigning GraphicElement display objects and instead invalidate only the GraphicElements that have their sharedDisplayObject reassigned (we were already invalidating when the exclusive display object is assigned).
Additionally I added a small check in the stroke property change event handler to make sure we invalidate parent size & layout only when the stroke weight is changed, which minimizes layout passes for skin state changes, where states would change stroke color/settings.