QBindableInterafce needs to support metatype != property type

We might have a property stored in a QPointer, but want to expose a plain pointer to the metatype system. To correctly handle assignments through the metatype system (e.g. in the QML engine), the type returned by the BINDABLE function should however match the type declared in the Q_PROPERTY declaration.

To enable this, we need to allow construction of QBindableInteraces which do the necessary conversions internally.