title: How a custom GWT Editor can know that a new edit begin (before the setValue)
body: <p>If we have an Editor with LeafValue subeditor.
  We can implement the interface ValueAwareEditor. with this interface with can implement a flush and a setValue.
  But the setValue is called after the driver has set all values in sub editor.
  How can we execute code before the driver set values in sub editor. (This may allow to clear the state of the editor).</p>
  
  <p>Is-there an equivalent of the flush method for an edit (or display) cycle in editor ?</p>
  
  <p>Thanks.</p>