[gtkmm] ButtonBox demo: Enable mnemonics in the buttons

[Kjell Ahlstedt <kjellahl src gnome org>]

commit aa1435c2ac4971611d79c361b721fc0a7aae1e79
Author: Kjell Ahlstedt <kjell ahlstedt bredband net>
Date:   Sat May 14 10:38:01 2016 +0200

    ButtonBox demo: Enable mnemonics in the buttons
    
    Add mnemonic=true to the button constructors. Bug #766228.

 demos/gtk-demo/example_buttonbox.cc |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/demos/gtk-demo/example_buttonbox.cc b/demos/gtk-demo/example_buttonbox.cc
index 15bac44..42680ad 100644
--- a/demos/gtk-demo/example_buttonbox.cc
+++ b/demos/gtk-demo/example_buttonbox.cc
@@ -83,10 +83,10 @@ Gtk::Frame* Example_ButtonBox::create_button_box(bool horizontal, const Glib::us
   pButtonBox->set_layout(layout);
   pButtonBox->set_spacing(spacing);
 
-  Gtk::Button* pButton = Gtk::manage(new Gtk::Button("_OK"));
+  Gtk::Button* pButton = Gtk::manage(new Gtk::Button("_OK", true));
   pButtonBox->add(*pButton);
 
-  pButton = Gtk::manage(new Gtk::Button("_Cancel"));
+  pButton = Gtk::manage(new Gtk::Button("_Cancel", true));
   pButtonBox->add(*pButton);
 
   pButton = Gtk::manage(new Gtk::Button());