Use Json to save and load particle effects

Note that the ParticleEmitter (BufferedReader reader) constructor is removed as the emitters are created by Json (in the ParticleEffect class).
As a result the ParticleEmitterBox2D (World world, BufferedReader reader) constructor is removed as well.

I tested this change using the particle editor and all seems to be working properly.

Do we need some kind of file converter to update the old particle files into the Json files?