QtProtobuf: make static const propertyOrdering constinit'able

The propertyOrdering static data member injected by Q_PROTOBUF_OBJECT is currently a std::unordered_map. However, it is neither dynamically-sized nor does do we expect many entries.

A hash-map which allocates each of its elements in a separate memory allocation is therefore overkill. If we don't need it's features, though, then all its drawbacks kick in, to wit:

1. dynamic initialisation causing program startup slowdown even if never used
2. dynamic destruction causing program shutdown slowdown even if never used
3. SIOF / SDOF

The staticMetaObject declared by Q_OBJECT and defined by the moc-generated code, otoh, is optimized for maximal sharing and minimal runtime initialisation.

The propertyOrdering member should also be given this kind of treatment.

Acceptance criteria:

• propertyOrdering is Q_CONSTINIT'able
• minimizes relocations to the minimum possible
• is trivially destructible, so it doesn't run code at program shutdown

A separate question is whether the json-name has to be stored in UTF-16, or whether UTF-8/US-ASCII aren't sufficient (halves string storage space).