The separates it from the LAYER_SELECTOR class - deciding
what color a layer is (say) is separate to managing the
actual selection of the layer. For example, sel_layer.cpp
only needs the presentation logic. This also makes it eaiser
to compose rather than inherit.
Additonally, break out the layer pair swatch function
to this class.
This will also be needed by the layer pair manager UI.
Relates-To: https://gitlab.com/kicad/code/kicad/-/issues/15227
db457f52fa