1) a mini-shell for command-line processing (a separate module)
     Audio::TagLib::Shell
     >open <file>
     >...
     >close
2) a GUI-based frontend
3) setArist(Audio::TagLib::String) -> setArist(PV)
4) overload q("") => sub{ shift->toCString(); } for Audio::TagLib::String
5) overload q("") => sub{ shift->data(); } for Audio::TagLib::ByteVector
6) implement q(=) for some classes