Unify the (multi-)transaction representation through import and export

There are currently at least 3 representations of transactions in the code base:
- from the importer
- from the DB ORM
- from the GUI

Those should be unified where possible (the ORM might have to remain the ugly little duckling) to simplify putting data into, pulling data out of the database. New importers might be especially hard (or redundant) to write without such a representation.