Generalize "behaviors" for all objects

[zackthehuman]

There are too many implementations of the same concept of object behavior in the code base. Enemies have "brains", Projectiles have "motions", Particles have an implicit "motion", Items have "effects", etc. These are all basically the same thing, with some minor differences, implemented in completely different ways.

There is no good reason for this fragmentation aside from the desire to experiment with different techniques.

As we move toward unifying the object system it will be a good idea to come up with an implementation of the core concept behind these different "behavior" implementations.