Add source field in Torrent creator. Closes #7965.

This field is often used for cross-seeding between (private) trackers.

src/base/bittorrent/torrentcreatorthread.h

@@ -43,6 +43,7 @@ namespace BitTorrent
         QString inputPath;
         QString savePath;
         QString comment;
+        QString source;
         QStringList trackers;
         QStringList urlSeeds;
     };