Submobjects of VMobject.

I believe, it would be a good idea if VMobject were more tolerant to having general Mobjects (non-VMobjects) as submobjects.

Rationale:
1. Mobjects, together with all its submobjects can be, for example, moved around.
2. It would be nice to being able to Mobject.add() some sort of anchors to a VMobject.
3. This way, if the vmobject is moved or rescaled, the anchors are automatically updated.
4. For example, to get a circle center, I am using VMobject.get_center(). However, during ShowCreation, VMobject.get_center() changes its position.
5. Worse than that, when you move a circle, for example, Circle:arc_center is not updated. Where it a submobject, it would be (hopefully) always updated.
6. Almost the same can be said about Line:start and Line:end.

Is it a good idea to make VMobject more tolerant to non-VMobject submobjects? I can implement it... if people think it is good.