fdo#84938: convert EPP_LAYOUT constants to enum

Change-Id: Ibf43bf9ca241c6286659a16906a45bbafa798748
Reviewed-on: https://gerrit.libreoffice.org/11961
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>