Fixup QGuiApplication::sync to flush the QWSI queue

Change-Id: If4cedeb886e912f622a66b2b3374d6260cffc32a
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/gui/kernel/qguiapplication.cpp

@@ -3002,6 +3002,7 @@ void QGuiApplication::sync()
             && QGuiApplicationPrivate::platform_integration->hasCapability(QPlatformIntegration::SyncState)) {
         QGuiApplicationPrivate::platform_integration->sync();
         QCoreApplication::processEvents();
+        QWindowSystemInterface::flushWindowSystemEvents();
     }
 }