MonthShortToString and DayOfWeekShortToString don't change the
underlying object. Those are just getters and can be declared `const`.

# Conflicts:
#	src/components/datetime/DateTimeController.cpp

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.