fdo#84938 replace PPT_LAYOUT_ constants with enum

Change-Id: I3c77e7e551bd4b657cb58fdfeead73d50441e890
Reviewed-on: https://gerrit.libreoffice.org/13437
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>