Obey power cooldowns; introduce "combat_style"; collision bugfixes

Closes #1283 

Notes:
- a348daf removes the usage of some enemy definition keys. Our alpha_demo game uses a few of these. We'll now use the power-specific cooldown instead. The old keys are:
 - cooldown_melee_phys
 - cooldown_melee_ment
 - cooldown_ranged_phys
 - cooldown_ranged_ment
- 80227bb removes passive_attacker from enemy definitions. We're not using that key in any of the current games. It's been replaced by combat_style, which has syntax defined in this commit.