title: Display selected sub-menuItem in NSPopUpButton
body: <p>I have filled my NSPopUpButton with menu and submenus.</p>
  
  <p>When I select a menuItem on the root menu (so, not a sub-menuItem), it propery displays the menuItem selected. And when I re-click on the popUp button, I get the selected menuItem under the mouse.</p>
  
  <p>But if I select a sub-menuItem, it doesn't display it. And if I re-click on the popUp button, I don't get it under the mouse. Any time I click on the button, I have to look for the selected item diving in the menu hierarchy.</p>
  
  <p>Any solution? </p>