Improve Lookup for QObject

The main reason for having propertycaches is that property lookups on the metaobject are rather slow - and they still are in Qt 6. While it should be possible to speed them up in Qt 6, we still won't match the speed of the property caches.

However, we can already skip property lookups if a QV4::Lookup is used, which caches the property data. Unfortunately, the qobject Lookup is somewhat limited and monomorphic. We ought to collect more data about Lookup transitions, and we probably will need a poly- or even megamorphic lookup for QObjects.