Say Hi to Merchant-Chan

Added a new NPC, Merchant-Chan. (For now.) As her name implies, she is a vendor and she buys your... trophies you found in your adventures. Of course, she sells stuff too.

Here is what the trading GUI looks like:

A couple things to note:

• Drag n Drop or double clicking on the item both work

• Currency is now an "item" and is visible in inventory and trading screens

• There is no need to directly drag currency onto the buy/sell window. Instead, the amount required for the exchange is automatically calculated and used. For example, if you drag an item worth $50 onto the "Items to give" window, then $50 is automatically added to the "Items to receive" window.

• You cannot pay with what you do not have:

That's pretty much it for the trading window. It should be easy and straight forward to use, although it was a huge PIA to implement.

Another change is that "seeds" no longer have infinite uses, but have to be brought individually:

This is the more intuitive behavior, but also a step towards implementing things like ammo for range weapons, and items with "charges" and limited uses effects.