title: QDockWidget is closed if main window is minimized
body: <p>I'm using Qt 4.7 on Windows 7 Ultimate 32 bit.</p>
  
  <p>The <code>QMainWindow</code> of my program has a <code>QDockWidget</code>. I've noticed that if I minimize the main window by the minimize button on the title bar, after restoring it the dock widget is closed. I didn't write any support for a feature like this!</p>
  
  <p>How does this happen and how to prevent this?</p>
  
  <p>Thanks.</p>