[Development] QProperty and library coding guide

[Ulf Hermann]

> Would it be sensible to make this a configure feature: if C++20 is explicitly enabled, use it? It’s in the standard after all.
> But auto-detecting it is perhaps unnecessarily fragile.

No, we should just static_assert for [[no_unique_address]] being 
available on the platforms where we expect it. That is, anywhere but on 
certain MSVCs.

If we make it a configure feature, we open the door to BIC problems as 
we might accidentally turn it off in some builds.

Ulf