Split the SceneGraph from the "state graph", and make the RenderLayer a property of Renderables

[Tomcc]

Currently a GameState is also an object.
This is not ok, a GameState should be a loose collection of objects that doesn't depend on the structure of the Scene Graph, usually.
Eg. a child of the Viewport should be allowed to be in a different GameState than the viewport itself, or the VP should belong to both states.

Also, manually adding Renderable to queues is error prone and not flexible enough, so it would be better to build the layers by traversing the scenegraph when something changes.

After the refactoring:

• remove all uses of setChildrenVisible