New enemy AI engine

[olofson]

Implement the behaviors needed for new enemies, similar to those in XKobo, in a modular, parameterized form, that lends itself for configuration via the theme parser or similar. (See #158.)

General guidelines

• One class only, so we can keep a pool of objects in a simple and efficient manner
• Cover player, enemies, bullets, explosions etc, to avoid special cases
• Hardwired states:
  • (Unused)
  • Alive
  • Exploding
• Arbitrary internal states:
  • Multiple sets of parameters
  • Inheritance; define states as diffs from other states
• Variables:
  • Position
  • Velocity
  • Health
• Events:
  • Spawned, enemy seen, flock seen, enemy died, damage, variable changes, etc
  • Events can change state
  • Events can modify variables
• Parameters/features (some of these are already in cs.[ch]):
  • Collision groups
    • Collision mask
    • Physics behavior
  • "Team" groups
    • Chase/evade behaviors
    • Flock behaviors