If no parent is provided the Windows system file picker uses either the current
foreground window or the desktop. The decision depends on the thread id of
the foreground window. To make sure the document window is used as parent this
is now also sent to the system file picker as it was already done for the
internal file picker.

Change-Id: Id589cbc1f91db30e065175aaea42ef1512ffb1b9
Reviewed-on: https://gerrit.libreoffice.org/24635Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>