What this commit does is keep the same code logic while moving to
GeglColor. Yet it's not **really** space-invaded yet. What we need to do
now:

1. Take into account the image space, and this is what we must navigate
   through, in particular for various representations of RGB or HSV.
   I.e. that if the active image is in anyRGB, the RGB values shown must
   be within anyRGB. Right now, everything is still shown/used as sRGB
   (even though it's properly retrieved and transformed to the target
   space thanks to GeglColor).
2. Show space info to make things clear and explicit, by adding some
   label somewhere.
3. Allow to switch between image and softproof spaces, regarding
   out-of-gamut display. I.e. that while RGB/HSV must be shown within
   the image space (assuming it's anyRGB), we may want to show
   out-of-gamut area (pink areas) within the softproof space. This may
   mean adding a checkbox. Or maybe simply taking into ...