Since save and export are separate activities we need to keep track of
image dirtiness for both of them. For this purpose, add a
'export_dirty' member to GimpImage which has the same semantics as
'dirty', but for export. Set it to clean whenever a document is
exported by whatever means. Do this with a new function
gimp_image_export_clean_all(). Also add gimp_image_is_export_dirty().