Group Combat improvements

* The first kill to join the group now spreads experience to the other
group members.

* A message will inform you when you get experience for a non-group kill
that still shares. For example, if someone killsteals you, or if you're
both fighting the same monster but not grouped.

* The message for group experience is only sent if you actually gain
experience. You won't see a message for mobs too weak to give you
experience.

* The room-based time before group membership expires is now automatically
adjusted based on the strength of mobs being killed within. Base group
time is 30 seconds for Tusked Skeletons and below. Mobs above
100 hp or of higher difficulty give higher group times. For example, Mollusks
will be 2 minutes and change. Avars will be 1 minute 20 seconds.
Thrashers have a special 2 minute bonus, giving them a group time of
4 minutes and 20 seconds, due to the hazardous nature of their room.
The group time is done this way so that it will automatically adjust
if mobs change - during an event, for example, or if we change spawns around.