convert SW_TAB #defines to enum

Change-Id: I8f2c63dc381416af1eb74794a1bbc3417eb272d6
Reviewed-on: https://gerrit.libreoffice.org/11936
Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>