Take over functionality from 'KDE5FilePicker::addCustomControl'
that has not been implemented in qt5 one yet.

Change-Id: I9690e163b3b739bfec6813d9a4e1e0170b730187
Reviewed-on: https://gerrit.libreoffice.org/68056
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
(cherry picked from commit c5947393)
Reviewed-on: https://gerrit.libreoffice.org/72653