Flesh out NSDocumentController subclass (REnamer, ec996a1)

This finishes the refactoring, giving us a nicer separation of responsibilities between the application control, the document class, and now the new NSDocumentController subclass.

As noted in the code comments in the later, this could later be expanded to support the "re-opening" of "documents".

Signed-off-by: Greg Hurrell <greg@hurrell.net>