Consistant rendering order for particles

This is related to particles sorting, but instead of sorting individual particles, make it possible to be consistent with the rendering order of sets. So that user could affect:

1) If all particles (of system / emitter / particle) are rendered before or after some model.
2) If all particles (of emitter / particle) are rendered before or after another particles (of emitter / particle).

One example of 1) is EmitterShapes.qml. Increasing models opacity shows how particles are sometimes rendered before and sometimes after the model (video1).

One example of 2) is Fire.qml. After changing 0 -> 10 into emitters directionVariation z, sometimes smoke is over and sometimes under the fire (video2).

Try to solve this for 6.1.