20 improve particle emitter a

Particle Effect creation now takes place only once. I had to add a Clock.schedule(_update) to ParticleSystem.start() so that the program could re-use each effect.

Obviously this has the side effect that the game can only use one of each type of effect at the same time. That's alright for now (and we get a serious performance and memory gain from only instantiating one of each type of effect) but if we want to use more effects in the future, we should add some kind of add_new_effect() method to ParticleEffects.