title: WPF treeview get path of selected node
body: <p>I have the following treeview (for example):</p>
  
  <p>-tasks
    -task1
       task2</p>
  
  <p>let's say that the selcted node is 'task2'.
  how can I get the full path to this node usin MVVM? it should be 'tasks.task1.task2'.</p>
  
  <p>thanks,
  itay</p>