border_width from 12 to 6, as the rest of dialogs

I know the old GNOME HIG mentions extra spacing for queries, but
I’ve never understood why -- not even GNOME itself respects that rule.
It just looks inconsistent and wastes space.

Change-Id: I181f800957f69b0943369791e460c7273b6b08f6
Reviewed-on: https://gerrit.libreoffice.org/11658
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>