Add new webUI API. Closes #6457.

Now getting piece information for a specific torrent is possible via:

* Returns an array of states (integers) of pieces in order. Defined as:
  "0=not downloaded", "1=downloading", "2=downloaded".
  GET /query/getPieceStates/<torrent_hash>

* Returns an array of hashes (strings) of pieces in order:
  GET /query/getPieceHashes/<torrent_hash>

src/base/bittorrent/torrentinfo.h

@@ -91,6 +91,7 @@ namespace BitTorrent
         QByteArray metadata() const;
         QStringList filesForPiece(int pieceIndex) const;
         QVector<int> fileIndicesForPiece(int pieceIndex) const;
+        QVector<QByteArray> pieceHashes() const;
 
         using PieceRange = IndexRange<int>;
         // returns pair of the first and the last pieces into which