When modifying txns involving foreign currencies, cases appear where the txn currency doesn't match the splits currecies.

Suppose you make:

- env LC_ALL=C gnucash --nofile
- create two US$ accounts us1 and us2
- create a euro account e
- in e's register, transfer 1 € to account us1, setting exchange rate of 1.2 -> in us1' register you'll see a 1.2 value. The txn has currency €
- in us1' register, change the e account to us2 -> since the txn currency is euro, now you have a € txn with two $ splits

I had this situation using autocompletion. I begin writing the description, and autocompletion gives me something that seems interesting, but then I realize the currency of the 2nd account was wrong, so if I change the 2nd account _after_ the autocompletion, the txn currency is already set to the currency of the txn used for autocompletion, and the user could have weird behaviours because he/she doesn't imagine that the txn has a currency different from the one he thinks he has set.

I suggest a two step solution:

- In autocompletion, don't set the txn currency until the txn is saved.
- Anyway, in modifying a txn, if the final tnx has no split with the txn currency, the txn currency should be changed to one of the currencies of the splits.