Item Edit GUI reference bug

Fixes wizzomafizzo/starcheat#60

Line 82 assigns the reference passed into init to self.item so all
changes to self.item are passed back to the initial caller. Fixed by
doing a deepcopy.

Haven't taken a closer look to see if there is a better way to fix it so consider this a temporary fix but it might spawn a more permanent one later. Thanks for working on the add and remove options.