title: how to add line number to QTreeView?
body: <p>I'm trying to code a simple c++ IDE based on Qt. Like any language IDE, a line number is very useful in the code editor. </p>
  
  <p>I'm using <code>QTreeView</code> for the editor and I'd like to add this line number feature in. Looks like <code>QTreeView::header()</code> only returns the Horizontal header to me. How can I <strong>get the vertical header</strong> to set the line number in <code>QAbstractItemModel::headerData()</code>?</p>