[Oracle] Replace vendored QtJson with native QJson for set import

- Drop the vendored oracle/src/qt-json/json.{h,cpp} implementation
- Switch SetToDownload and importCardsFromSet from QList<QVariant> to
  native QJsonArray/QJsonObject
- Release set JSON data after import in the save sets page

Took 20 minutes

format.sh

@@ -22,7 +22,6 @@ libcockatrice_* \
 exclude=("libcockatrice_rng/libcockatrice/rng/sfmt/" \
 "libcockatrice_utility/libcockatrice/utility/peglib.h" \
 "oracle/src/lzma/" \
-"oracle/src/qt-json/" \
 "oracle/src/zip/" \
 "servatrice/src/smtp/")
 exts=("cpp" "h" "proto")