Bug 440844

SUMMARY
When importing a statement, KMM checks if the account key, as provided by the importer, matches any account number or any account's StatementKey property. If none is matching, it then asks user to choose one by hand and sets the StatementKey value, so that in the future it can find the account automatically. 

However, the same account can be referenced by different IDs, depending for example on whether the statement was generated for a Checking Account or a Credit Card. Another use case is using different format, e.g. OFX and CSV.

STEPS TO REPRODUCE
1. Import a statement that references an account A formatted as A'
2. Assign A' to an existing account A
3. Import another statement that references account A formatted as A"
4. Assign A" to the same existing account A
5. Import another statement that references an account A formatted as A'

OBSERVED RESULT
KMM will ask again to manually find the A' on the list of account, despite having previously done so.

EXPECTED RESULT
KMM should remember both A' and A" as valid StatementKeys for the account A and not bother user anymore.