AoE overhaul

I spent the whole Sunday with overhauling the code for AoE attacks. It is now much more extensible for different attack patterns than just points and circles. I also implemented attacks which target a ground location instead of a specific opponent, which works both for mobs and for players.

However, the procedural spell generation system doesn't create any spells with that feature yet. Both kinds of AoE attacks are now visualized by the client during their preparation-time, so players can dodge attacks more intuitively.