[1.2.1] Add a flag to exclude certain transactions from statistics (e.g. bank transfers or refunds)

In some cases it may be useful to keep certain transactions out of the statistics (e.g. not to record a transfer or refund as "income"). For this purpose, a new flag in the transaction form view may be useful.

If the flag is set (the transaction is excluded from the statistics), it will be visible only in the account balance, but not in the balance and distribution.