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.

Eliminate the temporary buffer, do I/O right into the WritableBuffer.