[Nodes] - Expandable MixRGB Node

Reposting a proposition from 2019 GSoC thread on devtalk: devtalk.blender.org/t/improve-cycles-eevee-for-procedural-content-creation/7186/191

Currently to mix multiple color inputs we need additional nodes to handle mixing process and building desired hierarchy - as every mixRGB node can handle only two inputs.

Proposition goes with expandable lists of RGB inputs with mask/alpha/fac. Every input gets its own blending mode options, just like it works in almost every layer stack in 2D software. Arrows on the right are for reordering layers in a stack without messing with input links. Optionally reordering can be done with drag'n'drop.

There is no loss of functionality, because user still can build tree-like node layout with it.

The basic node should contain only two inputs, and as a user will add them, the next available input should be added to the bottom of the stack.