UB (invalid casts) in invokeOnGadget()

We found in QtProtobuf an invalid cast from QObject* to QProtoBufMessage*¹. QProtoBufMessage does not inherit QObject, so this cast is invalid.

It turns out that invokeOnGadget() also does cast from the Q_GADGET void*, which, too, isn't a QObject.

I can't say that I've ever seen ubsan complaints about this, but I see calls to QObject::d_ptr and QObject::thread() in QMetaMethodInvoker, so I don't see how this can't be UB.

¹ https://codereview.qt-project.org/c/qt/qtgrpc/+/550655/comment/007ce129_c588b70f/

Investigate whether we can split the QObject and Q_GADGET code-paths with the latter not casting to QObject* on the way already in Qt 7 or what else we can do to avoid the UB here.

Then fix QtProtobuf, too.