title: How to add submenu to MenuItem
body: <p>I'm trying to add submenu to a <code>MenuItem</code> which exists in a popup menu in system tray. Is there any way to achieve this? I've found some solutions about submenus but they use <code>JMenuItem</code>, and <code>TrayIcon</code> only accepts <code>PopupMenu</code> which only accepts <code>MenuItem</code>s. </p>
  
  <p>Trying to achieve this with <code>MenuItem</code>:</p>
  
  <p><img src="https://i.stack.imgur.com/zqFAa.png" alt="Image"></p>