Now that RefPtr is really a std::shared_ptr<>, we should use
std::weak_ref<> instead.

Note that a std::weak_ptr<> tells you nothing about whether
the underlying GObject is still alive, which Glib::RefPtr did.
It just tells you whether our std::shared_ptr<> still holds
a reference to it. That's why I removed one of the checks in
tests/giomm_listmodel/main.cc.