Add gimp_procedure_config_begin_export() and end_export() which
are wrappers around begin_run() and end_run() and additionally
pretty much completely manage GimpMetadata handling.

A GimpProcedureConfig can provide boolean properties "save-exif",
"save-xmp" etc. in order to have them automatically managed by
begin_export() and end_export(). This also restores the feature of
overriding the procedure's saved default values with export
preferences, but not the values from the last export, like it
used to be in 2.10.

Move gimp_image_metadata_save_prepare() and save_finish() (which are
now completely handled by GimpProcedureConfig) from libgimpui to
libgimp, but keep their declarations in the libgimpui header. Not
perfect, but not finished either.

Also fix gimp_image_metadata_save_prepare() to set the affected
GimpMetadataSaveFlags to 0 when the image has no metadata at all.