title: in GWT, How to add a row on top of header of celltable?
body: <p>This is very interesting, how can we add a row on top of header of celltable in GWT?</p>
  
  <p>Ok, Say,GWT Header only support sorting, but i want to do other stuffs on each of the column as well (such filter, hide, ....)</p>
  
  <p>So i want to add a row on top of header of celltable. On the row, there r many button, each button is corresponding to a column. When user click a button then they can do other stuffs like filter or hide.</p>
  
  <p>Example:</p>
  
  <p>Button1 - Button2 - Button3.</p>
  
  <p>Header1 - Header2 - Header3.</p>
  
  <p>Cell 11 - Cell 12 - Cell 13.</p>
  
  <p>Cell 21 - Cell 22 - Cell 23.</p>
  
  <p>more cell....</p>