GtkMenuButton: disable focus-on-click

As general rule, all buttons that launch a menu should
not grab focus on click, because otherwise when the menu
is closed the focus goes back to the button instead of
the previously focused widget, which is the one the user
was interacting with.

GtkScaleButton and GtkVolumeButton set focus-on-click
to FALSE for this same reason.

Fixes #2557

gtk/gtkmenubutton.c

@@ -621,6 +621,7 @@ gtk_menu_button_init (GtkMenuButton *menu_button)
 
   add_arrow (menu_button);
 
+  gtk_widget_set_focus_on_click (GTK_WIDGET (menu_button), FALSE);
   gtk_widget_set_sensitive (GTK_WIDGET (menu_button), FALSE);
 
   context = gtk_widget_get_style_context (GTK_WIDGET (menu_button));