title: Undo and Redo in WYSYWYG contentEditable in SWT doesn't work
body: <p>I'm doing a WYSYWYG HTML5 editor With SWT by setting the <code>contentEditable</code> attribute of the body tag to <code>true</code>.</p>
  
  <p>When I'm executing some commands like <code>document.execCommand('bold')</code>, it works perfectly. But when I try to undo an operation with <code>document.execCommand('undo')</code> nothing happens. I don't know if I have to set any undo manager or to do something like that. Can you help me please ?</p>