Changes to the particle effect

I've made these changes to make it less hardwired to the rest of the system:

  1. It now follows the new PhysicsUpdateAction paradigm instead of having its own update routine.
  2. PhysicsWorld.addParticleSystem is now PysicsParticleSystem.addToWorld.

    As a user you're only hit by the second point.

    There are still hardwired chunks of code related to it left, but I think I can iron them out as well… I'll leave that for another day though :slight_smile: