fix import torrent with "Keep incomplete torrents in:" ticked

* also had to account for "Append the label of the torrent to the save path",
  but again, this was only an issue when "Keep incomplete torrents in:" is
  selected

* 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.