Remove hard-coding of display fields from billingBlock.tpk

This change basically takes the logic out of which fields to show from smarty & hands it back to php where it can be manipulated by hooks. This is a small step change as part of making the payment processor code less hard-coded. A separate ticket will cover giving payment processors more control over allocating the fields

The following need to be assigned to the tpl

{$paymentTypeName} (assumes conversion of payment_type_id in the payment processor table to an option name)

{$paymentFields} - array of the names of payment related fields (e.g 'credit_card', 'expiry_date') - to display

{$billingDetailsFields} array of the names of billing related fields (e.g first_name, last_name to display)

• the second 2 effectively define fieldsets & allow fields to be added or removed more easily - note that the actual field definition is in QF

Parts
1) change to billingblock.tpl - rather like
https://github.com/eileenmcnaughton/civicrm-core/commit/e664cdb70a89efa73195929758d5cbb9056ac52e

2) alter relevant functions to assign the tpl vars mentioned above

3) convert payment_type_id to an option value (may have to do this first)