title: Issue with a BorderLayout and imaging
body: <p>Could you please take a look at the code below to suggest how to correct it to make it works? I tried to use different browsers but result is the same: nothing happens. There is just a blank screen:</p>
  
  <pre><code>public class GXTApp implements EntryPoint {
  
      public void onModuleLoad() {
  
          Viewport viewPort = new Viewport();
  
          final BorderLayout borderLayout = new BorderLayout();
          viewPort.setLayout(borderLayout);
  
          BorderLayoutData westData = new BorderLayoutData(Style.LayoutRegion.WEST, 200, 150, 300);
          westData.setCollapsible(true);
          westData.setSplit(true);
  
          BorderLayoutData centerData = new BorderLayoutData(Style.LayoutRegion.CENTER);
          centerData.setCollapsible(false);
  
          ContentPanel navPanel = new ContentPanel();
          ContentPanel mainPanel = new ContentPanel();
          viewPort.add(navPanel, westData);
          viewPort.add(mainPanel, centerData);
  
          RootPanel.get().add(viewPort);
      }
  }
  </code></pre>
  
  <p>Thank you for ideas how to fix it</p>