fdo#84938: replace HELPMODE_ constants with enum

Change-Id: I44be5567e84cdabd8b10771ea37e28b8a88cc23e
Reviewed-on: https://gerrit.libreoffice.org/12333
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>