libgimp: make gimp_image_metadata_save_prepare() suggest GimpMetadataSaveFlags

Return flags based on what metadata is actually present in the image.
Returning of a suggested value for GIMP_METADATA_SAVE_THUMBNAIL needs
support from gimp_image_metadata_load_prepare() and is still missing.

Port all plug-ins to use the new API, the suggested values are however
overridden by parasites and whatever special code was devised for the
individual plug-ins. This needs to be fixed.