Now that Song::GetDuration() is used (which considers start_ms and
end_ms), we don't need to override the Tag's duration value.

This invalidated the std::string instance, and thus broke the CUE
parser (commit 322b0616).

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.