Cleaner menu selection handling

• No interest in actions for MenuBar and Menu - only in MenuItem.
• Define a new subclass of MenuItem, MenuButton with two methods

  public boolean postEvent(Event evt) {
     if ((evt.id == Event.ACTION_EVENT)
         && action(evt, getLabel()))
           return true;
     return super.postEvent(evt);
  }
  
  public boolean action(Event evt,
          Object what) {
     return false;
  }