PlaylistEdit: postpone UpdateQueuedSong() when adding multiple songs

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.

src/command/QueueCommands.cxx

@@ -28,6 +28,7 @@
 #include "ClientFile.hxx"
 #include "Client.hxx"
 #include "Partition.hxx"
+#include "BulkEdit.hxx"
 #include "protocol/ArgParser.hxx"
 #include "protocol/Result.hxx"
 #include "ls.hxx"
@@ -73,6 +74,8 @@ handle_add(Client &client, gcc_unused int argc, char *argv[])
 		return print_playlist_result(client, result);
 	}
 
+	const ScopeBulkEdit bulk_edit(client.partition);
+
 	const DatabaseSelection selection(uri, true);
 	Error error;
 	return AddFromDatabase(client.partition, selection, error)