Add flag GIMP_METADATA_SAVE_COLOR_PROFILE to GimpMetadataSaveFlags and
initialize it from gimp_export_color_profile() in
gimp_image_metadata_save_prepare().

Adapt all plug-ins to use the bit from the suggested export flags and
pass the actually used value back to
gimp_image_metadata_save_finish().

This changes no behavior at all but creates hooks on the libgimp side
that are called with the context of an image before and after the
actual export, which might become useful later. Also, consistency
is good even though the color profile is not strictly "metadata".