fdo#84938 replace PPT_PLACEHOLDER_ constants->enum

Change-Id: I4110fab17f42a747ec55b27445f28bdef873ef86
Reviewed-on: https://gerrit.libreoffice.org/13447
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>