Improved trade screen

I changed the trade screen to match the inventory screen aspect. In the process, I migrated old C code to a C++ class heriting from the inventory class. With arrows, you can inspect the trader's items and your own items. With ENTER or items hotkeys, you select the items you want to buy/sell. The projected remaining gold is printed in the bottom of the screen. You can accept the transaction with the generic action key 'e' or cancel it with ESCAPE.