We used to set a private _q_foreignWinId property on QWindow when
creating foreign windows, and this was the mechanism which we then
passed the foreign winId to the platform plugin.
With c585802e94 this was removed,
since we now were passing the winId through via explicit QPA
APIs, and since 0c6911e5cd removed
the ability to explicitly destroy() a foreign window.
But when closing a QWindow, we destroy both the window itself,
and all its children, including foreign windows. In this case
we still want to support recreating the foreign window, for
example when the parent window is shown again. To enable this
we restore the _q_foreignWinId private property, but keep
the limitation of not being able to explicitly destroy a
foreign window.
Pick-to: 6.7 6.5
Fixes: QTBUG-124160
Change-Id: Ia885ba9f043e64fb21eedd2b4c344679726f1b5c
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
988039729f