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/DatabaseCommands.cxx

@@ -30,6 +30,7 @@
 #include "util/Error.hxx"
 #include "SongFilter.hxx"
 #include "protocol/Result.hxx"
+#include "BulkEdit.hxx"
 
 #include <assert.h>
 #include <string.h>
@@ -92,6 +93,8 @@ handle_match_add(Client &client, int argc, char *argv[], bool fold_case)
 		return CommandResult::ERROR;
 	}
 
+	const ScopeBulkEdit bulk_edit(client.partition);
+
 	const DatabaseSelection selection("", true, &filter);
 	Error error;
 	return AddFromDatabase(client.partition, selection, error)