Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

Remove them as parameters from `db_selection_print()`.

Prepare to allow more complex expressions.

Closes #172

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

Some database plugins don't use a Storage (e.g. UPnP), and with this
plugin, DatabaseDetachSong() can crash.

Wrapper for protocol/ArgParser.cxx.

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.