Feature Request: Canceling accepted local trades

Hello!

I was trading with a person who lives nearby and I initially proposed a local trade, but we later decided to just trade via mail.

I accidentally clicked 'Accept Trade' though which moved the local trade into a completed state and there is no way to undo that. Also there's a little warning next to the trade saying that I need to apply the changes.

It would be nifty if there was a `cancel` button which would revert the transaction and put it in the same state as the rest of the canceled trades.

Thanks! : )