L3 Core: UserDataHandler

I would prefer enhancing Events to enable UserData maintenance, though 
that may require defining events types for cloning and importing and 
adding a DOMStringList userDataKeys attribute to Node.

If not, does there need to be an NODE_ADOPTED operation type.

NODE_DELETED is problematic.  However, mimicing 
DOMNodeRemovedFromDocument would be well defined for garbage-collected 
platforms.