I feel like I've seen a developer tool when digging around the Apple Dev Tools that generates the interface for installer apps like that for you... It's why almost every installer follows the same steps and looks almost identical. I want to say its called "Package Maker" or something like that... I think I found it in the same folder as "Icon Maker" - except I think Apple has since done away with that tool in favor of just having several app icons with special names the same way as they do for iOS...