Implement refunds

[simhnna]

Evert now and then there is a transaction that's going to be refunded (partially).

If you split the check at a restaurant, but you pay for all with your credit card and the others repay you in cash or by a transfer.

If you have to pay something, that you'll be reimbursed for in the future (travel expenses, expenses covered by insurance, ...)

I guess a new model needs to be added, that can Link two transactions together along with a specifier for the type of link.