Custom particle emitters

[nkoenig]

For !44 (merged), we started using https://github.com/squarefeet/ShaderParticleEngine for handling particles.
In order to have more control over certain issues, I suggest we provide our own way of handling particles.
The following problems have been found using SPE:
Depth sorting of particles
Material. SPE allows just a texture and no other data.
No support for color range images. SPE allows at most four elements in an array that interpolates throughout the particle lifetime.
Soft particles.
See !44 (merged) and !44 (comment 547859897) for reference.