Decide how to check required properties on C++ types with dynamic metaobjects

We check required properties defined in C++ via the property cache; but if a C++ type has a dynamic meta-object (e.g. via a proxy), there is no property cache.

While we can't really handle a dynamic property which claims to be required, maybe we could still check the static properties of an object for requiredness. Care is however required, given that a dynamic meta-object could also hide static properties