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.
* Refactor and document the Watchdog driver to make it more readable.
Fix the configuration of the behaviours configuration that was not properly implemented (but it didn't cause any side effect since the correct value was eventually set in NRF_WDT->CONFIG).
Fix the wrong interpretation of the reset reasons caused by implicit conversions of int to bool.
Don't use relative imports like `../foo.h` as those depend on the
relative position of both files. Rather than that use imports relative
to the `src` directory, which explicitly is part of the include
directories.
