title: How to make AppBar component from material-ui-next react to scroll events
body: <p>As per <a href="https://material.io/design/components/app-bars-top.html#behavior" rel="noreferrer">Material Design guidelines</a>:</p>
  
  <blockquote>
    <p>Upon scrolling, the top app bar can […] transform in the following ways:<br>
    - Scrolling upward hides the top app bar<br>
    - Scrolling downward reveals the top app bar<br>
    When the top app bar scrolls, its elevation above other elements becomes apparent.</p>
  </blockquote>
  
  <p>Is there any built-in approach to do this in material-ui-next or should it be considered as a new feature? Can you give a hint on how to achieve the animation of the AppBar component as described in the guidelines?</p>