Same behavior as QWidget, and allows platform plugins to maintain order of
native windows based on the QWindow hierarchy, instead of having to manually
keep track of window levels.
Change-Id: Iacc7e9ee2527f0737c9da6debc7cec101064f782
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>