title: UiBinder work with ToggleButton
body: <p>I'm liking the new GWT2 UiBinder, however, it's not clear whether certain things are achievable using the declarative UI style. </p>
  
  <p>For instance, ToggleButton only takes the image instances at construction time (no setters for up/down images). As I understand, UiBinder works in a JavaBean-like reflective way, where the assignable attributes are mapped to corresponding setters. Is this style possible with widgets like ToggleButton, where certain attributes have to be specified at construction time?</p>
  
  <pre><code>&lt;g:ToggleButton ui:field="myBtn"&gt;&lt;/g:ToggleButton&gt;
  </code></pre>