Fix equality comparison between schema.org data types with Qt6

The new QVariant comparison API not only requires operator== but also
requires an operator< to be visible (even if not used) in order to work.

We missed those, so objects never compared equal triggering way too many
reservation updates in Itinerary for examples.

Implementing this also exposed that properties with hand-writeen
property getters/setters weren't covered by the generated comparison
operators at all. Similarly, types with an inheritance depth of 2 or
greater weren't recursing all the way down in their comparison operators.
No idea why this never showed up in the unit tests with Qt5...