title: GTK3 MenuItem checkbutton + icon?
body: <p>A piece of GTK software in my company features a menubar with some menu items in the menu bar being check buttons, and/or having an icon beside their label. However the menu is created using <code>Gtk.UIManager</code> and <code>Gtk.ActionGroup</code>, which are now both deprecated and are causing issues. For this reason, we are trying to rewrite the menu bar without these classes.</p>
  <p>I've looked at the <a href="https://lazka.github.io/pgi-docs/#Gtk-3.0/classes/MenuItem.html#methods" rel="nofollow noreferrer">documentation</a> for the <code>Gtk.MenuItem</code> class, and it only seems possible to set the item's text. No way to place an icon inline, or make the menu item togglable like a check button.</p>
  <p>Is there another way to achieve this, say place a little HBox inside the MenuItem, or is this no longer possible with non-deprecated features of GTK3?</p>
  <p>Thanks.</p>