Ok, here it is. I did this in Objective-C / Cocoa, so it’s Mac OS X only. If there is enough interest, I can look at Windows and / or Linux too.
I’d like to support animations, but I need more information on how those work.
It watches the files you’ve opened for changes. So the idea is that you use this alongside your pixel editing app.
The code is on GitHub:
https://github.com/fogleman/GameFrame
Here is the download: