Wrapping more methods of MapObject and ObjectGroup in python to make it possible to use them in python plugins

Wrappers added for ObjectGroup::objectCount(), ObjectGroup::objectAt(int index), MapObject::name(), MapObject::rotation(), visible(), and setName(), setRotation(), setVisible().

More in https://github.com/bjorn/tiled/issues/513

Hope I did it right, I have small experience with git.