Expenses: Add payment-type "SEPA direct-debit"

[mamayer]

I'd like to add expenses, that where paid via SEPA direct-debit mandate, but there is no "direct-debit" or "SEPA" in the list. The option "bank transfer" doesn't suite this case, because a bank transfer is a push-process while direct-debit is a pull-process.

Related to adding SEPA direct-debit as a payment-solution via stripe #1676