title: How does the PopupWindow get notified of touched outside events?
body: <p>The documentation states that <code>PopupWindow</code> will be informed of touch events outside of its window when 
  <code>setOutsideTouchable(boolean touchable)</code> is set to true.
  How is popupwindow informed? I don't see any listener like <code>setOnOutsideTouchListener</code> etc to receive that information.</p>
  
  <p>Example</p>
  
  <pre><code>PopupWindow popup = new PopupWindow();
  popup.setOutsideTouchable(true);
  //now what..how to receive those touch outside events?
  </code></pre>
  
  <p>Thanks.</p>