title: GXT Grid Checkbox Header Select/Unselect All
body: <p>Working with GXT and using a checkbox selection model.  I notice that when every row is checked, the checkbox header will also be checked, but the checkbox header has no functionality of its own.  </p>
  
  <p>How can I tell the checkbox header to select/unselect all checker rows when clicked? </p>
  
  <p>Thanks for the help!</p>