Add an option to add a reversing transaction to the one(s) currently selected: - add negateValue() to MyMoneySplit which does exactly that. - add reverse() to MyMoneyTransaction which negates value of all its splits, effectively reversing the transaction. - refactor slotDuplicateTransaction(reverse bool) to optionally handle the reversing. Assuming typical use case being to transfer the reversed amount to an income/expense category, the reversed transaction has the same date as the original. - add Icon::Reverse