Gtk::MenuItem: Fix add_accel_label()

Make sure accelator keys are shown right-aligned, by calling
gtk_menu_item_set_label().

https://mail.gnome.org/archives/gtkmm-list/2018-February/msg00006.html

gtk/src/menuitem.ccg

@@ -40,17 +40,8 @@ MenuItem::MenuItem(const Glib::ustring& label, bool mnemonic)
 
 void MenuItem::add_accel_label(const Glib::ustring& label, bool mnemonic)
 {
-  auto pLabel = manage(new AccelLabel(label, mnemonic));
-
-  //Labels are centered by default, but in menus they should be left-aligned.
-  pLabel->set_halign(Align::START);
-
-  add(*pLabel);
-
-  //Note that we have to unset the accel_widget in  MenuList::remove() to avoid
-  //a memory leak due to the extra reference:
-  pLabel->set_accel_widget(*this);
-  pLabel->show();
+  set_label(label);
+  set_use_underline(mnemonic);
 }
 
 void MenuItem::set_accel_key(const AccelKey& accel_key)