QInAppProduct should expose price and currency separately

Currently, QInAppProduct only has a QString price which contains the price as string in its localized form.

This prevents (makes unneccessarily hard) any kind of automated processing (e.g. track revenue) in the app.

There should be a
qreal price (needs a different name as price is already taken, or better a quint64 with "micro euros")
QString currency with the currency code (e.g. EUR)