* glib/glibmm/value.h: Add Value_RefPtrBoxed<>.
* tools/m4/class_opaque_refcounted.m4: Add _IS_REFCOUNTED_BOXEDTYPE, which can
be used together with _CLASS_OPAQUE_REFCOUNTED. It generates a *_get_type()
function and a Glib::Value specialization. A Glib::Value specialization is
required, if the C++ class is used in _WRAP_PROPERTY.