window: set GTK_STYLE_CLASS_TITLEBAR to custom titles

When a custom title is added to a GtkWindow, add the
GTK_STYLE_CLASS_TITLEBAR to it, so the theme can apply the default rules
for its style.

https://bugzilla.gnome.org/show_bug.cgi?id=706045

gtk/gtkwindow.c

@@ -3505,6 +3505,9 @@ gtk_window_set_titlebar (GtkWindow *window,
   if (visual)
     gtk_widget_set_visual (widget, visual);
 
+  gtk_style_context_add_class (gtk_widget_get_style_context (titlebar),
+                               GTK_STYLE_CLASS_TITLEBAR);
+
   gtk_widget_queue_resize (widget);
 }