border_width from 12 to 6, as the rest of dialogs

Second pass.

Change-Id: Ic4702d2ca7a9d54ee4712dcd073006f16b2d5d1d
Reviewed-on: https://gerrit.libreoffice.org/11669
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>