Use underlying_type member directly

`LTUnderlyingType` served as a intermediate type for libtorrent 1.1 and
1.2 and is obsoleted now.
Also add helper to convert to underlying type.

src/base/bittorrent/torrentcreatorthread.cpp

@@ -188,7 +188,7 @@ void TorrentCreatorThread::run()
             , [this, &newTorrent](const lt::piece_index_t n)
         {
             checkInterruptionRequested();
-            sendProgressSignal(static_cast<LTUnderlyingType<lt::piece_index_t>>(n), newTorrent.num_pieces());
+            sendProgressSignal(toLTUnderlyingType(n), newTorrent.num_pieces());
         });
 
         // Set qBittorrent as creator and add user comment to