/**
* Tells the DropDownAction that it should listen to mouse events on a given
* ToolBarManager. Used to display the Menu when the mouse is clicked.
*/
public IAction listenToMouseOn(IToolBarManager tbm) {
final ToolBar tb = ((ToolBarManager)tbm).getControl();
tb.addMouseListener(new MouseAdapter() {
public void mouseDown(MouseEvent e) {
ToolItem ti = tb.getItem(new Point(e.x, e.y));
if (!(ti.getData() instanceof ActionContributionItem)) return;
ActionContributionItem actionContributionItem = (ActionContributionItem) ti.getData();
IAction action = actionContributionItem.getAction();
if (action != DropDownAction.this) return;