Notifications menu is hidden when one clicks on the header

Steps to reproduce:

• open the notification menu
• click on the menu between "notifications" and "rss feed" (or any white part of the menu)
  The menu is closed.
  This is frustrating because the menu is hidden when user fails to click correctly on one button of that menu.
  This might be the cause of XWIKI-15029 (to confirm).

Note: this behavior is already fixed on the notification events. If you click inside of the events, the menu is not closed. All we have to do is to extend this behavior to the whole menu.