title: Jupyter notebook button widget: Remove all click handlers?
body: <p>It seems (correct me if I'm wrong), that when I invoke <code>button.on_click(callback)</code>, the object <code>button</code> being an instance of <code>ipywidgets.Button</code>, the function <code>callback</code> is added to a list of click handlers. How can I access this list?</p>
  
  <p>I would actually like to clear this list. If I could <em>find</em> the callback list (I can't!), I could apply <code>button.on_click(callback, remove=True)</code> for all registered callbacks.</p>
  
  <p>Is there a better/more idiomatic way than the above to remove all click handlers from a button widget? My handlers are generated programmatically and I don't want to have to store references to all of them if I want to delete them later...</p>