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/PlaylistCommands.cxx

@@ -26,6 +26,7 @@
 #include "PlaylistFile.hxx"
 #include "PlaylistVector.hxx"
 #include "PlaylistQueue.hxx"
+#include "BulkEdit.hxx"
 #include "TimePrint.hxx"
 #include "Client.hxx"
 #include "protocol/ArgParser.hxx"
@@ -67,6 +68,8 @@ handle_load(Client &client, int argc, char *argv[])
 	} else if (!check_range(client, &start_index, &end_index, argv[2]))
 		return CommandResult::ERROR;
 
+	const ScopeBulkEdit bulk_edit(client.partition);
+
 	const PlaylistResult result =
 		playlist_open_into_queue(argv[1],
 					 start_index, end_index,