Only send TorrentCompletedMessage if we have downloaded anything.
Prevents messages being sent in error when adding torrents that already exist on disk etc.
Removed some muting of events.
Refactored the torrent parts.
- Fixed setting file priorities. Closes #68.
- Instead of querying libtorrent for every request, an internal thread keeps the torrents updated and only the in-memory list is returned when getting torrents.