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.cpp

@@ -246,6 +246,21 @@ QVector<int> TorrentInfo::fileIndicesForPiece(int pieceIndex) const
     return res;
 }
 
+QVector<QByteArray> TorrentInfo::pieceHashes() const
+{
+    if (!isValid())
+        return {};
+
+    const int count = piecesCount();
+    QVector<QByteArray> hashes;
+    hashes.reserve(count);
+
+    for (int i = 0; i < count; ++i)
+        hashes += { m_nativeInfo->hash_for_piece_ptr(i), libtorrent::sha1_hash::size };
+
+    return hashes;
+}
+
 TorrentInfo::PieceRange TorrentInfo::filePieces(const QString& file) const
 {
     if (!isValid()) // if we do not check here the debug message will be printed, which would be not correct

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