Glib::TypeTraits<>: Avoid warnings when compiling with -Wcast-qual.
Glib::TypeTraits<>: Avoid warnings when compiling with -Wcast-qual.

* glib/glibmm/containerhandle_shared.h: Move const_cast<CTypeNonConst>(ptr)
from the TypeTraits< Glib::RefPtr<T> > specialization to the
TypeTraits< Glib::RefPtr<const T> > specialization, thus avoiding warnings
when compiling with -Wcast-qual. Bug #704640.

johne53/MB3Glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm