Let's use std::string in the core. Notably, I'd like to make
the numerous main() functions mostly independent of Qt. Some
things will have to remain, such as argument parsing, of course.
This changes the API: instead of returning an error code and
taking a pointer to the actual return-value, return an
std::optional<std::string>> that is set if the function succeeds.
Returning an empty string in the error case might be simpler,
but oh well...
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
c6cd10a43f