title: Capture window focus in event in Enaml
body: <p>Can I execute code when a window comes into focus (becomes the active window, e.g. when the window above it is closed)? I hoped windows would just have focus event or attribute, but that doesn't seem to be the case.</p>
  
  <p>There are focus events in Qt. If Enaml doesn't offer this, what's the easiest way to access these underlying Qt events?</p>
  
  <p>I'd like to be able to do something like:</p>
  
  <pre><code>enamldef MyWindow(Window):
      focus ::
          do_stuff()
  </code></pre>