Make QVariant::data() and ::constData() part of the public API

QVariant has some public functions for internal use, which are not part of the public API. This makes them unsuitable in customer code since their usage is unsupported:

void *data();
const void *constData() const;
const void *data() const;

It would be nice if these functions were made part of the public API as well, to allow direct access to the variant's data member.