Allow changing the order of the files to be downloaded for seqential mode

When in sequential mode, the files are always downloaded in the order they are present in the torrent file. It would be nice to have an option to put some files ahead of the others, as the original order doesn't always make sense. For example, for torrents with multi-volume rar archives, the first file, "volume.rar", usually comes last on the list, with the second "volume.r00" volume being the first on the list instead.