Convenience functions for the Colormap editor

[algernon]

Better still (for me, at least) would be to allow the user to assign a colour per "key type". The result would be a very fast way of getting something visually useful, very quickly indeed. This could just be a "key type colours" menu item.

Is your feature request related to a problem? Please describe.

The current colormap editor works, but some common operations, like setting all keys of a layer to the same color, or setting a given group of keys to the same color are tedious to accomplish.

Describe the solution you'd like

We would like a few helper tools that would make the above operations easier.

• [ ] Fill layer: Fill the whole layer with the selected color
• [ ] Color groups: Color the selected key group (alphas, numbers, modifiers, etc) with the same color

Additional context

Based on ideas given in #318.