add pooling support to particle effects

* particle effects in explosions are now using pools
* Explosions now directly require a TextureRegion instead of a Texture
* Explosions only check sibling explosions (getParent().getChildren() vs
getStage.getRoot().getChildren()). This allows explosions to be placed
in to a Group
* NPE protection for particle effects
* Added a ParticleEffectPool which does not reset effects on obtain,
unlike the badlogic ParticleEffectPool