Update UML documentation

Add explicit Window --<> Graphene composition.
Restore Object class, aggregate Object into SceneNode.
Inherit Camera, Entity, Light from Object.
Introduce SceneManager class with basic rendering methods.
Add sequence diagram draft.

Well, current architecture looks pretty much alike orge3d, I hope ogre
developers won't be mad.
I'd also like bad things to happen to umbrello devels. Producing nice
looking diagrams umbrello unleashes a world of hurt to those who use it.