Move FloatingWindow::setupWindow() into a reusable function

So other QWindow can use this

src/private/WidgetResizeHandler_p.h

@@ -69,6 +69,8 @@ public:
 
     static int widgetResizeHandlerMargin();
 
+    static void setupWindow(QWindow *window);
+
 #ifdef Q_OS_WIN
     static bool handleWindowsNativeEvent(FloatingWindow *w, const QByteArray &eventType, void *message, Qt5Qt6Compat::qintptr *result);
 #endif