When spawning enemies, don't block tile before checking requires_status
Fix spawn powers not working when their initial target was blocked
Use cooldowns from power definitions for enemy power cooldowns

This obsoletes the related cooldown keys in enemy definitions.
Obey power cooldowns for map events that trigger powers
Replace passive_attacker with combat_style in StatBlock

Instead of being a boolean, combat_style has three possible values:
- default: the existing behavior for creatures
- passive: only enters combat when attacked (i.e. passive_attacker=true)
- aggressive: is always in combat
Merge pull request #1284 from dorkster/fixes

Obey power cooldowns; introduce "combat_style"; collision bugfixes