Empty currently, but code will be added song.

Manipulates the playback range of a queued song.

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

Don't use global variable.

Works around a build failure on Solaris because annoyingly, Solaris
reserves the name "queue".  This rename was pending anyway.

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.

Fixes regression from commit e96779de.

Finally restores an important feature that has been broken for several
months when the PlayerThread started working with Song copies instead
of pointers to the Queue's Song instances (commit e96779de).