Make sure bankID is assigned when importing transactions

The bankID member of a split is the ultimate identifier to detect
duplicate transactions. Yet if empty it is not used and transactions of
the same import session are falsely detected as duplicates.

This change assigns a bankID in case it is empty based on the account
id, the post date, the payee name, the memo text, and the amount which
makes the bankID unique for the import session.

BUG: 419975
FIXED-IN: 5.0.9