[API] add registerStyleSheet to public Skin API?

[Jeanne Waldman]

I didn't realize until now that registerStyleSheet wasn't in the public 
Skin API. It's only in SkinImpl.
I'd like to add it to the Skin's APIs so that custom component 
developers can register skin
definitions for their custom components to the base skins, like simple.
This way if the end user sets the skin to simple, the custom components 
will have some
styling. If we didn't do this, the custom components would have no styling.
Thanks,
Jeanne