title: GWT's CellTable loading bar won't display while refreshing contents
body: <p>I do the following:  </p>
  
  <ol>
  <li>Create the CellTable (loading bar
  displays) </li>
  <li>Put data in it (loading bar disappears, data appears)</li>
  </ol>
  
  <p>I now want to:</p>
  
  <ol>
  <li>Clear data and show the loading bar </li>
  <li>Put more data in it (loading bar disappears, data appears)</li>
  </ol>
  
  <p>Has anyone done this? I've tried a few methods but they seem like hacks and haven't worked.</p>