Cleanups in Group and DataGroup. Change RTEs so not as many will have to be localized. Also, when mxmlContent is set, we clean up the old mxmlContent better and send out remove events.
QE Notes: Need to write tests when the mxmlContent gets set (remove events get dispatched).
Doc Notes: None
Bugs: SDK-18501, SDK-18064
Reviewer: Jason, Glenn
tests: checkintests, mustella Group, DataGroup, FxDataContainer, FxContainer, FxList, and GraphicTags tests