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

Add flag GIMP_METADATA_LOAD_COLORSPACE which defaults to TRUE, and in
gimp_image_metadata_load_finish(), assign AdobeRGB to the image if
Exif.Iop.InteroperabilityIndex says "R03". This is most likely very
incomplete because there are quite some other colorspace tags in
various parts of the image metadata.