Allow to define a palette for editor color schemes.

Motivation: I was looking for porting Gruvbox color scheme to the Editor. It defines a palette with colors for light and dark schemes. Then the specific colors are using palette references. As I've checked the editor settings does not support custom palette, only a handful basic colors are defined by default.

I though it would be easy and sufficient to enrich the XML schema to support some color reference mapping.

It is not goal to keep the defined palette on exporting the color scheme.