[gtkmm] Gtk::MenuItem: Fix add_accel_label()

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 336eb15a408063a93a501ee80470f54159075cfb
Author: Christian Schoenebeck <schoenebeck linuxsampler org>
Date:   Sun Feb 11 11:21:16 2018 +0100

    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 |   13 ++-----------
 1 files changed, 2 insertions(+), 11 deletions(-)
---
diff --git a/gtk/src/menuitem.ccg b/gtk/src/menuitem.ccg
index 5cd7242..dd2013b 100644
--- a/gtk/src/menuitem.ccg
+++ b/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)