Allow to create and remove avatars in groups.

- Akka is not very good at sending a lot of messages at once, m-boxes
tend to explode… Furthermore it is far more efficient, when possible to
group messages.
- Added AddAvatars and RemoveAvatars (note the plural) to add/remove a
large number of avatars of the same kind at once, both in Screen and in
RendererActor.