Add a constraint for bullet relative size to avoid too small
bullets which are hardly recognizable. Use the same 25% lower
limit what MSO uses.

Change-Id: Id956ecf3ec831c569188b944be58da03bf30a55e
Reviewed-on: https://gerrit.libreoffice.org/39696Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>