Convert CheckoutFlow, CheckoutPane, NumberPattern, PaymentGateway, PaymentMethodType, PaymentType, TaxType, TaxNumberType to attributes.

I started by defining TaxType and TaxNumberType attributes... But now I'm wondering... Perhaps it makes sense to prefix our attributes (just like our annotations, by Commerce, to be consistent with what was done for annotations...)

So TaxType would become CommerceTaxType for example.

Automatically closed - issue fixed for 2 weeks with no activity.