title: gwt custom widget with child elements configuration in UIBinder (like CustomButton)
body: <p>I've faced with a task to create custom widget, it has container behavior -- 3 panels inside.
  And i would like to use it in general UIBinder way like <a href="http://google-web-toolkit.googlecode.com/svn/javadoc/2.1/com/google/gwt/user/client/ui/CustomButton.html" rel="noreferrer">CustomButton</a></p>
  
  <pre><code> &lt;u:MyWidget&gt;
     &lt;u:image&gt;&lt;g:Image .../&gt;&lt;/u:image&gt;
     &lt;u:mainContent&gt;&lt;g:Panel.../&gt;&lt;/u:mainContent&gt;
   &lt;/u:MyWinget&gt;
  </code></pre>
  
  <p>Is it possible to define that custom child elements somehow or maybe it is predefined thing in the UIBuilder?</p>
  
  <p>Thanks in advance</p>