qvariant: Avoid dangling QObject pointers

When storing a QObject pointer or a QObject-subclass pointer in a QVariant, the raw pointer is stored in QVariant's QObject *Data::o member. Numerous times in the past this has caused crashes in user code when the pointer became dangling.

We should consider making QVariant safe by defaulting to supporting a weak reference.

Unfortunately, this cannot be done in Qt 5 without breaking binary compatibility.