title: QWidget::showMinimized() doesn't work
body: <p>On Ubuntu 13.04, if using <code>QWidget::showMinimized()</code> to minimize a window, I found that after restoring it by clicking the app icon the system taskbar, recalling the  <code>QWidget::showMinimized()</code> cannot work.</p>
  
  <pre><code>connect(minimumBtn,SIGNAL(clicked()),this,SLOT(minimumWin()));
  minimumWin(){
     showMinimized();
  }
  </code></pre>
  
  <p><code>showMinimized()</code> in <code>minimumWin()</code> doesn't work anymore if it has been called before (even the window is showed).</p>