add isFixedSize implementation

Yuhang Zhao

2023-12-05 d02f3b0bc32f711bf33a7fd288185963e9e96189

 src/quick/quickwindowagent_p.h

@@ -15,7 +15,7 @@
        void init();

        // Host
        QQuickWindow *host{};
        QQuickWindow *hostWindow{};
    };

}