Added an event propagation system on the avatar hierarchy.

- Events propagate an are consumed from the renderer through the screen
toward avatars.
- Also added width/height/depth shortcuts to the NumberSeq and
therefore Vector2 / Vector3.
- Added a way to multiply a point by the top-most model view matrix in
Space.
- Added an auto computation of the inverse top model view matrix in
Space.
- Added a way to multiply a point by the top inverse model view matrix
in Space.
- Added a backend for renderers, moved RendererNewt in it.
- Specified a set of events from it.