fix for importing multi-file torrents with only one file

A multi-file torrent with only one file (ie: a single file within a folder),
was being treated as a single-file torrent, making it impossible to import.
Multi-file torrent detection code was copied from libtorrent.  The information
is available in libtorrent (under torrent_info::m_multifile), however it's a
private member and I chose to go with copying the code that determines it,
rather than modifying a library qBittorrent depends on.