Blender Git Loki

Cleanup: Move Face Sets random color generation to its own function

This way we can change the color generation easily if we want to improve
it in the future. I also added more values to randomize a little bit the
saturation and value of the colors, as previously it was too easy to get
similar colors when creating new faces, forcing you to use the randomize
colors more than necessary.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D7042