Refactor SpriteEngine out of StochasticEngine

Refactor SpriteEngine out of StochasticEngine

Also add ParticleGroups which use only StochasticStates
Simplistic change for now, just to focus the API for the
particle system. ParticleGroup elements replace the particleStates
property on the system, and the term "group" is now used more
consistently.

Change-Id: I6456f9c521b8166ccd94ea953275557bcfbf6423
Reviewed-on: http://codereview.qt-project.org/4699
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>