Disconnect QWindowContainer::focusWindowChanged on destruction

Regression after 9e78256579.

Apparently the string based connection syntax is more lenient,
as with PMF-style connects, we (correctly) get:

 ASSERT failure in QWindowContainer: "Called object is not of the
 correct type (class destructor may have already run)"

when the QWidget destructor closes the QWindow and we get a
synchronous callback from the platform that we've lost the
focus window.

Pick-to: 6.7
Change-Id: I8f25b3b050e11913ec0f0941eaf831c88b4249f9
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>

src/widgets/kernel/qwindowcontainer.cpp

@@ -245,6 +245,9 @@ QWindowContainer::~QWindowContainer()
         d->window->destroy();
 
     delete d->window;
+
+    disconnect(qGuiApp, &QGuiApplication::focusWindowChanged,
+               this, &QWindowContainer::focusWindowChanged);
 }