Some refactoring: add gimp_metadata_get,set_colorspace() and a new
enum GimpMetadataColorspace which so far can be one of { UNSPECIFIED,
UNCALIBRATED, SRGB, ADOBERGB }. The setter is untested and I don't
know if it's doing the right thing, please review. Use the getter in
gimp_image_metadata_load_finish(), so complex metadata logic and
profile creation/setting are separated.
Copy a ton of logic from darktable and libkexiv2 and parse more
metadata tags which contain colorspace information, namely:
Exif.Photo.ColorSpace
Xmp.exif.ColorSpace
Exif.Nikon3.ColorSpace
Exif.Canon.ColorSpace