title: Merge two columns of a JavaFX TreeTableView
body: <p>Im trying to implement the <strong>Column Span</strong> of a <strong>TreeTableView</strong> Shown in the <em>Cell</em> Section of the TreeTableView wiki. But theres no documentation in how to do so.</p>
  
  <p>Can anyone help me with this one?</p>
  
  <p>Wiki: <a href="https://wikis.oracle.com/display/OpenJDK/TreeTableView+User+Experience+Documentation" rel="nofollow">https://wikis.oracle.com/display/OpenJDK/TreeTableView+User+Experience+Documentation</a></p>