plug-ins: verify that GimpColorConfig.rgb_profile is actually for RGB

and remove the RGB check from all callers of lcms_image_get_profile(),
the function now returns an RGB profile or NULL. This implicitly stop
checking that 'icc-profile' actually contains an RGB profile.
libgimpcolor: return an optional MD5 digest from gimp_lcms_create_srgb_profile()

pass NULL in most places, use the feature in the lcms.c plu-gin.