Make everything Entities again

[JeremyLeland]

The Actor vs Particle difference isn’t meaningful enough to warrant separate systems. Make everything entities, put them all in one list.

Particles can be entities with damage = 0. Can also add an isSolid property. Can add checks in update loop to avoid collision detection/response for entities without damage or that are not solid.

This also makes it easier for update, hitWith, etc to return lists of created Entities for the level to keep track of (since it doesn’t have to differentiate between particles and actors)

[JeremyLeland]

This would make it easier for our ship debris to bounce off the asteroids.