This is to allow new group chats to coexist with old group chats. We do
not rename everything in group.[ch] to conference, yet, because it's not
currently necessary, and a general internal API overhaul is due at some
point anyway.
It is still C code, so still compatible with C compilers as well. This
change lets us see more clearly where implicit conversions occur by
making them explicit.
This removes the global logger (which by the way was deleted when the first tox
was killed, so other toxes would then stop logging). Various bits of the code
now carry a logger or pass it around. It's a bit less transparent now, but now
there is no need to have a global logger, and clients can decide what to log and
where.
