[gtkmm/gtkmm-3-22] Gtk::MenuItem: Fix add_accel_label()

[Kjell Ahlstedt <kjellahl src gnome org>]

commit e5c8c2df67d0d7ec255055984e8f07d0f0fa0862
Author: Christian Schoenebeck <schoenebeck linuxsampler org>
Date:   Sun Feb 11 10:29:12 2018 +0100

    Gtk::MenuItem: Fix add_accel_label()
    
    Make sure accelator keys are shown, by calling gtk_menu_item_set_label().
    
    https://mail.gnome.org/archives/gtkmm-list/2018-February/msg00006.html

 gtk/src/menuitem.ccg |   13 ++-----------
 1 files changed, 2 insertions(+), 11 deletions(-)
---
diff --git a/gtk/src/menuitem.ccg b/gtk/src/menuitem.ccg
index 59ed513..1c3bf46 100644
--- a/gtk/src/menuitem.ccg
+++ b/gtk/src/menuitem.ccg
@@ -48,17 +48,8 @@ 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)