Add LCH to the color selectors, patch by Elle Stone and myself.

- Extend enum GimpColorSelectorChannel by LCH channels
- Support them in GimpColorScale, GimpColorScales and GimpColorSelect
- Did not yet remove the HSV channels until things are working 100% ok
- Change drawing in GimpColorSelect to be much faster, to compensate
  for babl_process() making the LCH modes pretty slow
- Clean up stuff in GimpColorSelect

This is WIP and should not be considered finished, biggest TODO is
displaying out-of-gamut values in GimpColorScales' spinbuttons.