convert PORTION_KIND constants to enum

Change-Id: I32d65cfd7a67cd7ebca0f99061293a9ac3398133
Reviewed-on: https://gerrit.libreoffice.org/11970
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>