Bug 492048 - Detect color space in Exif 2.21/DCF 2.0 option files

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.