Convert GimpRGB to CMYK using a color transform to the configured CMYK
profile instead of the naive gimp_rgb_to_cmyk().

Add gimp_color_frame_set_color_config() and call it on all color
frames in the GUI (color picker tool, cursor info, sample point view).
Keep a GimpColorTransform around that does the conversion.

Also color manages the frame's color area now (visible in the sample
point view), which was forgotten earlier. Addresses bug #467930.