Parse the R128_TRACK_GAIN comment string.

Call decoder_timestamp().  This is not necessary currently, but will
be as soon as we implement seeking.

This requires seeking to the end-of-stream, checking its granulepos,
and then seeking back to the previous file position.  We do this only
for local files.

Fixes regression from commit e96779de.

Don't use IgnoreError() when there's an overload that does not try to
give us one.

Leave that to the caller, allowing it to modify multiple items at a
time.

Fixes purging deleted songs from the queue after update.  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).

Move code from class Instance.

Merge the two loops into one, and eliminate the GSList.

Eliminate duplicate code.