[crossfire] Quests & such

[Nicolas Weeger]

Ok, I've just committed quest-related code.
Check the doc/Developers/quests file for info on how to use it and such.
It Worked During My Tests (tm), but of course it needs more testing to
be sure it works as intended :)

Note that this code does not currently let you mark a quest completed
when a monster is killed - you need to play with forces & such. Maybe
in a next version.

Also it doesn't handle party-related issues (a player completes a
quest, should other players in party complete it too?).

Nicolas