New actor graphics framework
:-)

- Added a new actor graphics framework.
- Defined the notion of Space, that is in fact the modelview/projection
matrices stack.
- The new actor framework is based on a model that is made of actors,
- And a specific RendererActor that acts as a rendering tree for
« avatars ».
- Avatars are a hierarchy of elements that define a space, event
interactions, and a renderer.