title: How to add a CustomJS callback for a Panel?
body: <p>How would I add a CustomJS callback for a Bokeh Panel? For example, say i want
  to change the text within a bokeh.model.Div based off the currently selected Panel in a Tab.
  My naive attempt is something like:</p>
  
  <pre><code>div = Div(text='Test Div', width=100, height=100)
  panel = Panel(child=plot, title=plot.title)
  callback = CustomJS(args=dict(div=div), code='div.text = "change successful!";')
  panel.js_on_event(events.Tap, callback)
  </code></pre>
  
  <p>however this obviously doesn't work, So i'm wondering if anyone can correct this example?</p>