Kept a bit on the trade shop mode.

- Fixed the crash when removing all of an item type when trading.
- Fixed the check for the item existence when Adding a trade.
- Made the trade price possibly different from the standard price
which is quite logical when you buy something full price but pay
less when trading it against other wares.
- Removed the debug trade from the actual game and added trades
in the debug shop instead.

The trade mode is almost functional now. Only need to show clearly
the trade conditions.