Add gimp_image_import_color_profile(), a GUI vtable entry
query_profile_policy() and a new dialog which returns the profile
policy and the profile to convert to. Get rid of the wrapper that
calls the lcms plug-in for that dialog, the plug-in is now completely
unused.

This commit doesn't add any new features, it's just the former lcms
plug-in dialog implemented in app/ (except the little fix that it is
now aware of linear vs. gamma images).