Impact of C++20 std::format on our code

C++20 added a new formatting library, roughly modelled after Python's. It supports user-defined types, user-defined formatting options, char and wchar_t, argument reordering, localization, and is based on format strings.

This library impacts Qt APIs on two levels:

1. enable Qt types for use with std::format
2. replace our existing formatting (QString::arg(), QTest::toString(), QDebug stream operators) with std::format
   • it seems to have everything we need for localisation
   • except char16_t support
     • but maybe there's a way to extend the library with char16_t support?