Check presence of observers and validators in owner

[SylvainCorlay]

In xproperty.hpp, properties call methods on the owner.

    template <class T, class O, class D>
    template <class V>
    inline auto xproperty<T, O, D>::operator=(V&& value) -> reference
    {
        m_value = owner()->template invoke_validators<derived_type>(std::forward<V>(value));
        owner()->notify(derived_cast());
        owner()->template invoke_observers<derived_type>();
        return m_value;
    }

If we check that the methods exist at compile time and only call them if it is the case, we can prevent the requirement on the owner to have certain methods.