Adding SpriteVisualElement, a light-weight, Sprited-based implementation of IVisualElement. This will be used for some FXG compiler optimizations.
Added setter for includeInLayout into IVisualElement so that DataGroup's virtualization code can use this, rather than cast it to a UIComponent. I also removed an unused method in DataGroup: _removeChild. We needed this before to remove children on a previously parented DataGroup, but now that we do the cleanup pro-actively, this is no longer needed.
Also a few ASDoc cleanups in IVisualElement, ILayoutElement, and FxDataContainer.
QE Notes: -
Doc Notes: -
Reviewer: Chet, Evtim
tests: checkintests, mustella list, datagroup, fxdatacontainer, buttonbar. Some mustella were failing, but I think it's just a mac thing.