Implement mainwindow example

Sine Striker

2023-12-05 f8975a7d449bac21a2fdbc1a8ca85d1f5b99b362

 src/core/contexts/win32windowcontext_p.h

@@ -33,6 +33,9 @@
        bool snapLayoutHandler(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam,
                               LRESULT *result);

        bool customWindowHandler(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam,
                                 LRESULT *result);

    protected:
        WId windowId = 0;