Removed vestigal [SCNScene sceneWithURL]...

…from [self readFromURL]
Doc saves camera viewpoints

In which we learnt to use NSData as [NSValue CATransform3DValue] can't
be archived by NSDictionary (obviously) or NSKeyedArchiver (not
obviously).
Also view menu should be per-doc not per-app.
Scene to Video registration sane, if far from perfect.