[sugar] Buddy-left working

[Morgan Collett]

The Presence Service should now emit a working buddy-left signal when
someone leaves a shared activity. Shared activities should also
disappear from the mesh view when everybody has left, although I'm still
chasing down some cases when this does not happen.

The code is still being improved, so ping me if you have any issues with
this. (or log a trac ticket on the presence-service component if I'm not
around...)

Regards
Morgan