title: Use a system icon in the setIcon method of QWidget
body: <p>I would like cross-platform a way to get a system icon, for example the folder icon <a href="https://i.stack.imgur.com/qBw25.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/qBw25.png" alt="folder icon"></a> in the <code>setIcon</code> method of <code>QWidget</code>. Something like this:</p>
  
  <pre><code>QWidget *myWidget = new QWidget;
  myWidget-&gt;setIcon(/*something to get a system icon*/);
  </code></pre>
  
  <p>Is this possible? If it is, how to do it?</p>