Change "Incomplete/temp folder" term with "download folder".
Allow to set "download folder" per torrent (in manual mode) and per category (in automatic mode).
Originally, it was just a wrapper for libtorrent::torrent_handle class, so it mimicked its name.
It was then transformed into a more complex aggregate, but the name was retained (just by inertia).
Unlike libtorrent::torrent_handle class in whose name "handle" means the pattern used,
it does not matter for qBittorrent classes and just eats up space in the source code.
Arguments include adding torrents as paused/started, skiping hash check, setting category, downloading in sequential order, downloading first and last pieces first, and skipping the 'add new torrent' dialog. Added TriStateBoolOption class for specifying options that don't have a default value when unused (e.g. add-paused). Also improved command line usage text to include more information, have better organization, and not exceed 80 columns in width. Also also added firstLastPiecePriority field to BitTorrent::AddTorrentData, and modified TorrentHandle so that if first/last piece priority should be on, it will be toggled on after the torrent's metadata has loaded.
