Implement new option -f: fresh start, overwriting Steca.conf.

Steca stores numerous state variables in .config/Steca/Steca.conf (under Linux). This can cause difficulties in reproducing reported issues, unless the reporter also provided his Steca.conf. As a remedy, I suggest to implement a new command-line option

    -f   Fresh start, with old configuration moved to Steca.conf.bak

If this option is chosen, then before any call to Qt we call a function from std::filesystem that moves Steca.conf out of the way. Upon that, the GUI should start with default settings.