Access some more data in a non-blocking manner

* Cache URL seeds to access in a non-blocking manner
* Provide non-blocking way to create magnet URI for torrent

PR #18572.

src/base/bittorrent/torrentimpl.h

@@ -327,6 +327,7 @@ namespace BitTorrent
         MaintenanceJob m_maintenanceJob = MaintenanceJob::None;
 
         QVector<TrackerEntry> m_trackerEntries;
+        QVector<QUrl> m_urlSeeds;
         FileErrorInfo m_lastFileError;
 
         // Persistent data