QtLocation shouldn't use QV4 internals

Qt Location currently uses QV4 internals to convert lists of gadgets into JS arrays (for instance in QDeclarativePolygonMapItem::path via the fromList helper). That is most likely not necessary in Qt 6; the engine can generally deal with lists of gadgets quite well.  Relying on V4 internals is also rather brittle, and the QML team would prefer if this low-level private API would only be used in declarative itself. Moreover, tooling has a much easier time dealing with specific types than with generic QJSValues.

Therefore, it should be investigated whether the usage of V4 API in QtLocation can simply be removed by instead returning plain QLists of gadgets instead.