title: How to use QStyle::standardIcon/standardPixmap with QStyle::StandardPixmap?
body: <p>In my code there is a bunch of calls which try to create <code>QIcons</code> from
  <code>QStyle</code> standard pixmaps, like:</p>
  
  <pre><code>QIcon groupIcon;
  groupIcon.addPixmap( style()-&gt;standardPixmap( QStyle::SP_DirClosedIcon ),
                       QIcon::Normal, QIcon::Off );
  groupIcon.addPixmap( style()-&gt;standardPixmap( QStyle::SP_DirOpenIcon ),
                       QIcon::Normal, QIcon::On );
  </code></pre>
  
  <p>While this works correctly, in that using the icon for a model's
  <code>Qt::DecorationRole</code> shows either an open or closed icon based on the item's
  expanded state, it has two issues:</p>
  
  <ol>
  <li>It's not hi-dpi friendly, and the icons are tiny</li>
  <li><code>QStyle::standardPixmap</code> is marked as obsolete, with <code>QStyle::standardIcon</code> being described as the preferred approach.</li>
  </ol>
  
  <p>I'm unsure how to translate the above code to <code>QStyle::standardIcon</code> though.</p>
  
  <pre><code>QIcon groupIcon( style()-&gt;standardIcon( QStyle::SP_DirClosedIcon ) );
  </code></pre>
  
  <p>works nicely for closed items, and looks great on hidpi. But I can't see how I would add the <code>SP_DirOpenIcon</code> state. There's no equivalent method like "<code>QIcon::addIcon</code>" like there is <code>QIcon::addPixmap</code>.</p>
  
  <p>What's the correct approach to take here, which is hi-dpi friendly and future proof?</p>