title: JavaFX8: display an image as disclosure node for childless TreeViewItems
body: <p>I have defined images for the two states of a disclosure node (collapse/expand) according to <a href="https://stackoverflow.com/questions/15521806/change-expand-and-collapse-image-treeview-javafx-2-2">Change expand and collapse image TreeView JavaFX 2.2</a></p>
  
  <p>But I would like to have a third image for folder nodes, which still haven't any children nodes. It should not possible to expand/collapse such nodes, but it should display an image too (i.e. a minus.png for expanded nodes, a plus.png for collapsed nodes and an empty rectangle for folder nodes without any children).</p>
  
  <p>Is it possible?</p>