Improve "info hash" handling

Define "torrent ID" concept, which is either a SHA1 hash for torrents of version 1,
or a SHA256 hash (truncated to SHA1 hash length) for torrents of version 2.
Add support for native libtorrent2 info hashes.

src/base/bittorrent/torrent.cpp

@@ -31,6 +31,8 @@
 
 #include <QHash>
 
+#include "infohash.h"
+
 namespace BitTorrent
 {
     uint qHash(const TorrentState key, const uint seed)
@@ -49,6 +51,11 @@ namespace BitTorrent
     const qreal Torrent::MAX_RATIO = 9999;
     const int Torrent::MAX_SEEDING_TIME = 525600;
 
+    TorrentID Torrent::id() const
+    {
+        return infoHash().toTorrentID();
+    }
+
     bool Torrent::isResumed() const
     {
         return !isPaused();