Add child property to custom container.

[Larik Ishkulov <gentro etersoft ru>]

Hi!

I have a container derived formm Gtk::HBox. How can I add new child property 
in to it. I didn't find any support for it in gtkmm.
I use gtk_container_class_install_child_property. Looks like it works, and 
even glade normally works with it. But I can't get access to it from widget.

Thanks.
Larik Ishkulov