fdo#85347 Make file picker modal again

Change-Id: I0e26b9e13e69ed49e7bbbc7f2ac0b04941ee1da7
Reviewed-on: https://gerrit.libreoffice.org/12974
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit e5a7e7249a32493d175ad94ee4e650320518d711)