title: JsTree json leaf detection
body: <p>I have a JsTree instance in which all childs are loaded async by a json call.</p>
  
  <p>How can I detect if a user has <strong>selected</strong> a leaf node? </p>
  
  <p>I've tried to use the success callback of ajax, but of course that also gets called if you simply open a Node with a "+". Is there an event like "child nodes loaded"?</p>