Use CMake's configure_file() functionality to generate the list of User Applications.
All the apps included in current versions of InfiniTime are enabled by default, but this can now be overridden by setting variables ENABLE_APP_XXX to True or False.
CMake CMP0140 is set to NEW to enable the return PROPAGATE functionality.
Watch faces can now be selected at buid time. It's implemented in a similar way than the selection of user apps, using a list of watch face description that is generated at build time (consteval, constexpr)
A list of "user applications" is built at compile time. It contains all the info needed to create the application at runtime (ptr to a create() function) and to display the app in the application menu. All applications declare a TypeTrait with these information.
When a new app must be loaded, DisplayApp first check if this app is a System app (in which case it creates it like it did before). If it's not a System app, it looks for the app in the list of User applications and creates it if it found it.
Those changes allow to more easily add new app and to select which app must be built into the firmware.
Switch to C++20 (and fix a few issues in SpiMaster.cpp and Watchdog.cpp.
