Add load png file for scan

[tehruttiger]

There have been many times when I wanted to test scan a specific stockpile image and I would have to comment/uncomment lines of code and hardcode the filename into the code. I should just have a load thing that opens a browse window for *.png starting in the Stockpiler folder and allow a user to pick an image (doesn't have to be cropped stockpile image) to scan. Would make building a modded icon set easy if you just had a pile of stockpile images gathered from other users.