Consider using manual transaction's label as custom label in operations merge

Use case: when I create a manual transaction (to plan my budget/balance) I set a label. Later, when I merge the manual transaction with the actual one from the bank, the label I set disappeared (the label of the new transaction is used).

When merging transactions we should consider using the manual transaction as a custom label (and use it if there is no custom label for the other transaction, as it is already the case).