Gtk::Object::_release_c_instance(): Unref orphan managed widgets
Gtk::Object::_release_c_instance(): Unref orphan managed widgets

g_object_run_dispose() unrefs a widget only if it has a parent.
Use g_object_unref() on all widgets without a parent.
Fixes #115

GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm