Qt6 QProperty should be markable as dirty manually

Many Projects are moved from Qt 5.15 to Qt 6, there are many Properties, defined over the "old" way. Since those are incompatible to the new Qt 6 Property system, one must implement a way to invalidate all QProperties depending on Qt5 Q_PROPERTIES as soon the latter changes. Currently, I don't see a way to do this without writing unintended hacks like
```
connect(this, &Class::qt5PropertyChanged, [&]

);
```

I would like to have either a hint in the documentation of the Property system or a function like `QProperty::invalidate()` / `QProperty::markDirty()`.