JPEG: Store/Retrieve CMYK profile via GimpImage
This patch builds on @Jehan's CMYK work and attempts to add the new gimp_image_set_simulation_profile ()
and gimp_image_get_simulation_profile ()
APIs to the JPEG plug-in.
On load, the CMYK profile will now be stored in GimpImage, and when exporting, the GimpImage CMYK profile will be considered rather than the default profile defined in Preferences.
Similar changes were applied to the TIFF plug-in via merge requests !681 (merged) and !627 (merged), as well as the PSD import plug-in with merge request !646 (merged)