ColormapOverlay should work more like colormap

[obra]

This is a follow-on from #1420 - I think that ColormapOverlay would fit better in Kaleidoscope if it were configured the same way as Colormap.

@EvyBongers's argument against:

It can certainly be done, but I feel that that defeats the purpose of the plugin. This is something that Colormap does already and does really well. I feel that Colormap-Overlay would typically apply colors to just a few keys, at they are applied on top of the active LED effect.

What I'm thinking about is mostly about making the API easier for users to understand. Having the two plugins configured with equivalent datastructures means there's less syntax for folks to learn, we don't need to provide the big comment showing off the key map layout, etc. If the colormaps and overlay colormaps worked the same, it'd also make it more straightforward to add support for colormap overlays to chrysalis.

I don't really object to having a "just overlay this one key" function, but for use cases like how we're handling numpad, I think it'd be nice to have the whole-keyboard level config.