Allow applications to use gtk-enable-mnemonics in 3.12

gnome-terminal is still using this setting, so we'll let
applications override it for another cycle. It is no longer
backed by a system-wide setting, though, and it will still
go away eventually.

This partically reverts 7e3a494fac

gtk/gtkmenushell.c

@@ -955,6 +955,7 @@ gtk_menu_shell_key_press (GtkWidget   *widget,
 {
   GtkMenuShell *menu_shell = GTK_MENU_SHELL (widget);
   GtkMenuShellPrivate *priv = menu_shell->priv;
+  gboolean enable_mnemonics;
 
   priv->keyboard_mode = TRUE;
 
@@ -965,7 +966,14 @@ gtk_menu_shell_key_press (GtkWidget   *widget,
   if (gtk_bindings_activate_event (G_OBJECT (widget), event))
     return TRUE;
 
-  return gtk_menu_shell_activate_mnemonic (menu_shell, event);
+  g_object_get (gtk_widget_get_settings (widget),
+                "gtk-enable-mnemonics", &enable_mnemonics,
+                NULL);
+
+  if (enable_mnemonics)
+    return gtk_menu_shell_activate_mnemonic (menu_shell, event);
+
+  return FALSE;
 }
 
 static gint