Since there is no way to retrieve the embedded native file dialog
from the Qt5 platfor abstraction, force the dialog in non-native
mode. This drops the 2nd option dialog completely.

While at it implement handling of the optional combo boxes in the
dialog.

Change-Id: I8d11e929a5f3b142dc0f5653fcc576231a1b0392
Reviewed-on: https://gerrit.libreoffice.org/60569
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>