title: ipywidgets dropdown widgets: what is the onchange event?
body: <p>I can register a handler to button.on_click in ipython notebook widgets, but I don't know how to do the same for a dropdown widget</p>
  
  <pre><code>import ipywidgets as widgets
  from IPython.display import display
  
  def on_button_clicked(b):
      print("Button clicked.")
  
  button = widgets.Button(description="Click Me!")
  display(button)
  
  button.on_click(on_button_clicked)
  </code></pre>
  
  <p>But for </p>
  
  <pre><code>choose_task = widgets.Dropdown(
      options=['Addition', 'Multiplication', 'Subtraction'],
      value='Addition',
      description='Task:',
  )
  </code></pre>
  
  <p>there seems to be only </p>
  
  <pre><code>on_trait_change(...)
  </code></pre>
  
  <p>if I register a handler with this, can I use it to access the value of the widget?
  I have seen examples with the handler and the widget belong to a subclass, and the handler can use self to introspect. But if I don't want to use a subclass, how does the handler know what widget was the target of the event.?</p>