title: How to listen for changed hidden attribute
body: <p>Whats the correct way to listen for a change on the hidden attribute in a Lit Element?</p>
  
  <p>e.g. an element like this:</p>
  
  <p><code>&lt;my-element ?hidden="${this.activeTab != 1}"&gt;&lt;/my-element&gt;</code></p>
  
  <p>I tried to define <code>updated(changedProperties)</code> but this does not trigger for a change in hidden.</p>
  
  <p>Note on the background of the question:<br>
  I am using leaflet in this element and it seems like i have to call <code>_onResize()</code> on my leaflet container element after i change the visibility.</p>