vlcrs-core: object: add Object::new()

This allows creating objects from existing objects (in a libvlc tree) or dummy objects (parent = None) for tests, and provide the Drop implementation to delete the objects when they go out of scope.

The implementation is specifically made to be too restrictive for now, which makes it unpractical to other use case than creating tests, since we cannot guarantee the correct usage of the object tree as it is otherwise.