Make the payment totals area handler extensible

There is a Views area handler in commerce_payment_handler_area_total.inc that defines a footer for the Payments table as specced in #841974: Payment UI. Currently it's hardcoded to provide a total and balance rows for a View consisting of payments of a single currency or multiple total rows for a View consisting of payments of multiple currencies. However, no one else can add information to this table right now, so we should make this extensible using some sort of a hook to collect additional rows for the area and order them accordingly.