Profiles from disk have a lower precision than those created from
scratch via lcms API. Ensure identical profiles by returning a
"loaded" profile from gimp_lcms_create_srgb_profile() (simply do a
save/load roundtrip in memory).
which returns a string meant to label the profile in the GUI, it's
either the profile's description, its model, or "(unnamed profile)" as
a fallback. Use the function instead of duplicating that logic
inconsistently and imcompletely all over the place.
Add functions to get a profile's description, manufacturer, model and
copyright, and use them instead of implementing the same 10 times.
Also add a GimpColorProfile typedef which avoids both having to
include lcms globally or using a gpointer instead (which looks bad and
non-descriptive in an API).
