Merge QVariant and QMetaType's builtin types into one class

At the moment there is two different classes that are used in this respect, QVariant actually uses the QMetaType classes's types, but there is a conflict regarding the user one and also there is no real need to have both. So merge the two into a single class.

Update:
Such an endeavor cannot be done until Qt 6 due to binary and source compabitility reasons.