Improve MPD compatibility

Add more commands / improve commands compatibility with the MPD specifications.

inc/lektor/commands.h

@@ -43,6 +43,7 @@ bool command_crop   (struct lkt_state *srv, char *args[LKT_MESSAGE_ARGS_MAX]);
 bool command_move   (struct lkt_state *srv, char *args[LKT_MESSAGE_ARGS_MAX]);
 bool command_shuffle(struct lkt_state *srv, char *args[LKT_MESSAGE_ARGS_MAX]);
 bool command_dump   (struct lkt_state *srv, char *args[LKT_MESSAGE_ARGS_MAX]);
+bool command_swap   (struct lkt_state *srv, char *args[LKT_MESSAGE_ARGS_MAX]);
 
 bool command_queue_list(struct lkt_state *srv, size_t c, char *args[LKT_MESSAGE_ARGS_MAX]);