Since KDE5FilePicker relies on the Plasma-specific KFileWidget
to be present for inserting custom controls, this doesn't work
on non-Plasma desktops, resulting in missing controls.

Therefore, use a plain (non-native) Qt5FilePicker for the non-Plasma case.

Change-Id: I163af89ecd7eab800d55baa2ce057802caed262c
Reviewed-on: https://gerrit.libreoffice.org/67106
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>