Impact of C++20 multi-threading library additions on our code

C++20 added new multi-threading classes:

1. std::jthread and std::stop_token (__cpp_lib_jthread, [wg21.link/P0660], [wg21.link/P1869]): thread class that join()'s in the destructor, and a way to cancel threads cooperatively
   • Qt already has solutions for these, but we may want to support stop_token in our APIs
2. Latches and Barriers (__cpp_lib_latch, [wg21.link/P1135], [wg21.link/P0666]): new threading primitives wrapping semaphores in higher-level concepts
   • The usual reflex would be to reimplement these as Qt types, but we shouldn't. If there're enough use-cases to warrant a q20 implementation, we should do that instead
     • but solve the problem of missing std::thread and std::future support on MingW first
3. Semaphores (__cpp_lib_semaphores, [wg21.link/P1135], [wg21.link/P0514]): Counting semaphore
   • Can probably be used to back QSemaphore, removing platform-specific Qt implementation
     • benchmark first
4. Waiting on every std::atomic (cf. QTBUG-105467)
   • First tests show that it's still slower than QMutex
     • should investigate why and fix std implementation(s) so we have a fighting chance to get off our own implementation of threading primitives in this decade

With the exception of <stop_token>, there appears to be nothing that would affect our APIs.