title: Rounded bottom on appbar
body: <p>I want to make an appbar with a rounded bottom, like so:</p>
  
  <p><a href="https://i.stack.imgur.com/eMW7A.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/eMW7A.jpg" alt="Rounded appbar"></a></p>
  
  <p>How would I go about implementing such an appbar? 
  I have tried reading up on the documentation for CustomPainter, but I don't feel like that is the way to go.</p>