This is a first commit to really getting rid of GimpRGB within core and
PDB/plug-in code. This will make color conversion reliability a lot better as
GeglColor will handle conversions for us. The goal is that we should keep origin
color space (for instance when picking colors in a GimpPickable, or when storing
in the FG/BG colors or in paletters) until the last second and convert at use
only.
It's still very much work-in-progress.