Do not allow payment methods to be used for terminal payments

1) Not all Payment payment methods can be processed this way. There is currently no way to let the system know which ones that are, so that should be added. Basic payment method should work, though.

2) When I try to add a payment using the terminal, it fails, simply because Payment's payment/payment method validation kicks in and says there are no available payment methods (which shouldn't happen either).

This requires more fundamental changes. Currently, we execute Payment payments in the redirect pane, which does not exist for terminal payments. This means we'll have to add one or two metadata properties to payment method controllers:
- "requires payer action": whether payments using the controller need the payer to perform actions during payment execution.
- "interrupts context workflow": whether executing a payment using the controller interrupts the context's workflow, e.g. by redirecting the user off-site.
If either of these are true, administrators cannot use this controller to add terminal payments.

This isn't working for the Basic payment method for me. I find that I getting these same two errors when I try to make a payment through the commerce payment screen.

Supporting terminal payments requires fundamental changes to Payment. Until those changes are made, let's make sure that Payment payment methods are not exposed at all.

Can you please confirm whether this works?

It works for me with the latest dev version.

Thanks for the reviews!