title: retrieve treeview data on double click
body: <p>I am trying to retrieve <code>treeview</code> data by double-click(<code>row-activated</code> signal).
  I have managed by now to do that by <code>changed</code> signal as:</p>
  
  <pre><code>self.view.get_selection().connect("changed", self.row_activated)
  def row_activated(self, selection):
      (model, iter) = selection.get_selected()
      print(model[iter][:])
      return True
  </code></pre>
  
  <p>but I can't managed it with <code>row-activated</code>:</p>
  
  <pre><code>self.view.connect("row-activated", self.row_activated)
  def row_activated(self):
      (model, iter) = self.view.get_selected()
      print(model[iter][:])
      return True
  </code></pre>
  
  <p>But, this is giving error:</p>
  
  <pre><code>TypeError: row_activated() takes 1 positional argument but 4 were given
  </code></pre>