convert SBX flag bits to type-safe enum

Change-Id: I18d5d6a27f06ee60a5cb3dc393bf05b51bba4817
Reviewed-on: https://gerrit.libreoffice.org/11070
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>