convert SWFMTFLD_ #defines to an enum

Change-Id: I5daf6436fd049410c4863a4b065c9abd95b5d361
Reviewed-on: https://gerrit.libreoffice.org/11928
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>