title: Add a checkbox to most parent tree element?
body: <p>I want to add a checkbox widget to the most upper parent tree element like shown in <a href="http://www.smartclient.com/smartgwt/showcase/#tree_checkbox" rel="nofollow">http://www.smartclient.com/smartgwt/showcase/#tree_checkbox</a></p>
  
  <p>How could I do this by just using GWT?
  I only found that I can create a Tree*Item* with a checkbox and add it to the Tree:</p>
  
  <pre><code>TreeItem item = new TreeItem(new CheckBox("box"));
  </code></pre>
  
  <p>But how can I create a box for the Tree itself?</p>