Implement a "bulk" edit mode that postpones both UpdateQueuedSong()
and OnModified(). This way, the playlist version gets incremented
only once. More importantly: when adding multiple songs to a queue
that consists of only one song, the first song that got added will
always be played next. By postponing this choice, all newly added
songs get a chance to become the next song. Fixes the second (and
last) part of Mantis ticket 0004005.
Add a new CommandResult code called "FINISH" which, unlike "CLOSE",
will attempt to flush the output buffer. This is a one-shot attempt;
it will do one write, and not try again.
This command was removed by commit 206392ad (MPD 0.16), even though it
was been proven useful for some very simple clients. On request, I
add it to the protocol again.
