connect(showMenu, SIGNAL(triggered()), this, SLOT(menuToggled()));

void MainWindow::menuToggled()
{
    bool toggled = true;
    if(toggled)
    {
        container->setCurrentWidget(menuWidget);
        toggled = false;
    }
    else if(toggled == false)
    {
        container->setCurrentWidget(mdiContainer);
        toggled = true;
    }
}