Changes based on Skinning Architecture spec. In SkinnableComponent, made _skinObject getter/setter private (rather than protected). Also cleaned up some documentation in there. Renamed attachBehaviors()/removeBehaviors() to skinLoaded()/unloadingSkin(). Out of this rename, I had to touch a few other files and found a few small bugs, like forgetting to call super.unloadingSkin(), forgetting to remove event listeners, and putting code in skinLoaded()/unloadingSkin() rather than partAdded()/partRemoved().