Fix nativeEvent() to support Qt6

[xxr0ss]

nativeEvent() fixed

[PaddleStroke]

You probably rather want something like

#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
bool FramelessWindow::nativeEvent(const QByteArray& eventType, void* message, long* result)
#else
bool FramelessWindow::nativeEvent(const QByteArray& eventType, void* message, qintptr* result)
#endif

So that doesn't break people using qt5

[xxr0ss]

@PaddleStroke Thanks for pointing this out!