[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

oracle/src/pages.cpp

@@ -560,6 +560,9 @@ void SaveSetsPage::initializePage()
 
         int setsImported = wizard()->importer->startImport();
 
+        // JSON data no longer needed after CardInfo objects are built
+        wizard()->importer->releaseSetData();
+
         if (setsImported == 0) {
             QMessageBox::critical(this, tr("Error"), tr("No set has been imported."));
         }