Particles, more class hierarchy work

* Added MovableObject, base class for all objects inside a level that
can move. Entities extend it, as well as
* Particles! Very basic at the moment, but at least it's something. You
can create them one-by-one, or add a ParticleEmitter to the level that
uses a ParticleBuilder to generate fancy particles nonstop.
* Camera extends a new class called "Chaser", which is also extended by
ParticleEmitter to provide a consistent way of following entities (->
now IChasable).