QtQuick.Dialogs: use URLs for interface with QPlatformFileDialogHelper

Depends on f5dbc876378ae58a7bdfe1e9664fc81caca18dfb and
5069e203d7aff7fb49bf746071f66bfd41c1ae89
Reverts commits 17ded06804576dfde1b19f82f168f7ceb09ec92c and
and d9751bc29d2f80b86eb237226ce832bf2e372f1d

Change-Id: Ibe2347f3b57e4e569bc4966735e9623dfd274628
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Kevin Ottens <kevin.ottens@kdab.com>