Add Import option to import "Price mark up" of an existing world

[Gabthelazy]

Pretty straight forward. Currently, importing a game will reset the existing "price mark up" of a world to a new one. Adding mods and updating to newer versions sometimes require importing and this usually break up existing trade routes a player might have. Adding the option to import the existing economy would definitely benefit trader type players or people who just want to peddle stuff while moving from one point to an other.

Personally, I find it very inconvenient and time consuming having to remake my trade goods spreadsheet every time I import.