[gtkmm] Fix the return type of Gtk::Widget::get_style_context() const

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 1b443da1f3b5b522bdc98f03071cad077e98bb19
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Thu Dec 12 09:14:56 2019 +0100

    Fix the return type of Gtk::Widget::get_style_context() const
    
    It shall return a RefPtr<const StyleContext>.
    See https://mail.gnome.org/archives/gtkmm-list/2019-December/msg00006.html

 gtk/src/widget.hg | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/src/widget.hg b/gtk/src/widget.hg
index c6e35853..b44fc244 100644
--- a/gtk/src/widget.hg
+++ b/gtk/src/widget.hg
@@ -502,7 +502,7 @@ public:
   _WRAP_METHOD(bool in_destruction() const, gtk_widget_in_destruction)
 
   _WRAP_METHOD(Glib::RefPtr<StyleContext> get_style_context(), gtk_widget_get_style_context, refreturn)
-  _WRAP_METHOD(Glib::RefPtr<Gtk::StyleContext> get_style_context() const, gtk_widget_get_style_context, 
refreturn, constversion)
+  _WRAP_METHOD(Glib::RefPtr<const StyleContext> get_style_context() const, gtk_widget_get_style_context, 
refreturn, constversion)
 
   _WRAP_METHOD(Gdk::ModifierType get_modifier_mask(Gdk::ModifierIntent intent), gtk_widget_get_modifier_mask)