a widget's window is transient for its parent's top-level window

When a widget's parent's window is not a top-level window, it should
find the top-level window before calling setTransientParent,
to avoid a warning (since a71e2851

).

Task-number: QTBUG-42464
Change-Id: I732691b0d40aba226470332426775d1bd4381009
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>