Container: Document several rarely used virtual functions
Container: Document several rarely used virtual functions

* gtk/src/container.hg: Add documentation for the following functions:
child_type(), forall(), composite_child(), [get|set]_child_property(). The
documentation comes from the C docs.

Bug #670206.

johne53/MB3Gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,GNOME/gtkmm