title: Vaadin Tree: Overlay or more icons
body: <p>does anybody have a solution for my problem? I need more icons for items in my Tree component.<br><br>
  Icons are useful for conveying item's node type, but I need to express its status or another properties. <br>It would be ideal if Tree had methods like addItemIcon or addItemOverlayIcon.<br><br>
  Thanks for advices.</p>