Deprecate QDate / QTime <-> Date conversions

As documented https://doc.qt.io/qt-6/qtqml-cppintegration-data.html#qdate-and-javascript-date , QDate and QTime usage in QML is extremely error prone, resulting in off-by-one bugs if one isn't super careful. While some "mitigation" exists (like only calling UTC-named Date functions), formatting would still return the local time, and so on.

I think these conversions should be outright deprecated, and removed in a ".pragma strict" QML. If the user needs to carry a

struct Date { int year,month,day; }

they can create their own value type. I don't see any value at having "convenience" which comes with so many traps attached.