Enhancement: shop price variable option and buy/sell event option

[PyroTech03]

Shops currently are very static. If we wanted to simulate any sort of economy, we currently have to make multiple copies of the same shop. This is time consuming and prone to errors.

I'd like to suggest some changes on the shop window that would go far in allowing users to simulate a dynamic economy in our games.
First, adding an option to tie a variable to a percentage change in listed prices. This could be one for both buy/sell or ideally a variable for buy and one for sell. These would look for integer and add/remove a percent of the price it would otherwise use in the shop.

Second, adding an option to tie to a common event on buy and on sell. This would allow us to create a common event to adjust the value of the variables we're using in our shops.

With these changes, we could simulate on a small degree an economy in our worlds at a basic level based on player actions of contributing to said economy. Thus allowing players to impact the world. With specifics able to be dictated by developers. This can be from having a fluctuating economy to allowing for a barter skill to allowing reputation with a given vendor.