Move away from GimpRGB to GeglColor

This is a branch where I'm working on fully moving away from GimpRGB, GimpHSV, GimpHSL and GimpCMYK. These would be replaced by GeglColor which is space-invasion friendly and which contains a few helper functions to easily grab color as RGB, soon CMYK, and maybe why not HSL/CMYK (see gegl!141 (merged)).

For the first commits, I added support in PDB to pass GeglColor objects on the wire and fixed #9790 (closed). There is much more to be done including updating all core code as well to use GeglColor. Palettes should have GeglColor (there was the discussion lately with @cmyk.student that in some palette formats, different colors may even use different color model/space), picked colors should keep the source image's space until the last second (converted to target space only when being used), and so on.