title: What is the 'icon' argument for ipywidgets.Button?
body: <p>I am trying to understand how to display buttons in an jupyter notebook, but the <a href="https://ipywidgets.readthedocs.io/en/latest/examples/Widget%20List.html#Button" rel="nofollow noreferrer">documentation</a> is a bis sparse on this item. </p>
  
  <p>It is mentioned there is an argument 'icon'. But what does it mean? What values can be used? Is there some documentation hidden somewhere else?</p>