title: GWT: How to add DockLayoutPanel inside another panel? Not the RootLayoutPanel
body: <p>Is it possible to add DockLayoutPanel inside another panel other than the RootLayoutPanel?
  If so, how would you do it?
  I attempted to add it inside HTMLPanel, and only north of the DockLayoutPanel shows.
  Then I tried it inside LayoutPanel, it failed again.</p>
  
  <p>I thought it would have work for LayoutPanel because RootLayoutPanel is a subclass of LayoutPanel.</p>
  
  <p>Any answers welcome.
  Thanks,</p>