Replace color phases with color ramp for Particles2D. (need some review/guidance)

Created by: Biliogadafr

I've tried to replace color phases for particles 2d with more convenient to use color ramp with dedicated editor. I'm completely not sure that I did everything correctly, especially from multithreading point of view, but at least it works. So it would be nice to hear some advices or ways of improvement of this patch. To do this task I made a copy of existing gradient editor from shader graph editor and do some modifications to it (I decided to try replace original one with modified version later). So what was done: -new resource for color ramp + editor -backward compatibility. Old coloring for particles 2d will be converted to color ramp (or to new color property if only one color phase was used before). -undo/redo support.