this.tree.setCanAcceptDrop( true );
this.tree.setDragDataAction( DragDataAction.MOVE );
// add action handler to the
this.tree.addSelectionChangedHandler( new MailfolderSelectionChangedHandler() );
this.tree.addDataArrivedHandler( new DataArrivedHandler() {
public void onDataArrived( DataArrivedEvent event ) {
tree.getData().setModelType( TreeModelType.CHILDREN );
tree.getData().openAll();