Impact of C++20 concepts on our code

C++20 added "Concepts Lite", a reduced version of "Concepts TS", itself a reduced version of the concepts that were, for a while, in C++11, before being yanked out at the last moment.

The impact of concepts on Qt has several axes:

1. Qt types should model corresponding std concepts where it makes sense. This should be checked at compile-time in the type's respective unit-test, and it should be documented (ex: QMutex models std::basic_lockable).
2. Qt function and class templates may want to constrain their template arguments (and document that) using either
   1. ad-hoc concepts: these concepts are completely self-defined without help from the std header <concepts>, which means they can be used in C++17, too (with std::enable_if). There's a danger here that each library will do the same thing and we get a world of NIH concepts, though.
   2. (directly or indirectly) std concepts
3. ...