Glib::Binding: Fix return type from dup_source() and dup_target()
Glib::Binding: Fix return type from dup_source() and dup_target()

Change from Glib::RefPtr<Glib::Object> to Glib::RefPtr<Glib::ObjectBase>.

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm