title: QTreeView merge some cells
body: <p>I have a custom QAbstractItemModel and custom tree view.<br>
  Is it possible to merge some cells in QTreeView?  </p>
  
  <p>It shold looks like this:</p>
  
  <pre><code>Num | Name      | Qty | .... |
  ----|-----------|-----|------|
  1   | Unit one  |  5  | .... |
  1.1 | Sub unit1 |  3  | .... |
  1.2 | Very very big string   |
  1.3 | Sub unit2 |  2  | .... |
  </code></pre>
  
  <p>Also, QTreeWidget::setFirstColumnSpanned() do not that is necessary.</p>