Handle German Sonderwurst in SEPA XML files

This patch series implements support for the German (non-)standard for SEPA XML files (#48), complete with per-creditor configuration.

Note: The fourth commit ("Emit XML header only when file generation is complete") is not strictly related to this functionality -- but it is necessary for the error message that can be generated by the code added in next commit to show up properly.

Also, most of the DAO updates in the first commit are not strictly necessary -- but as we need to update one of the DAOs, we better update all of them for consistency...