[Charts] Handle refunds

[dlebech]

When an item is refunded, the original expense is still the only thing showing up in expenses overview. This leads to an incorrect view of expenses.

1. The easiest option to fix this is to simply include all transaction amounts in the expenses sum charts, but ignore categories where the sum is positive (this should automatically exclude actual income). The problem with this approach is if the expense and refund transactions happen far apart. Then their grouped sums will be in separate months, and it won't work.
2. Add a way to tag something as a refund to something else. This seems to be the proper approach, as it will also allow partial refunds in a good way.