Me porting Runestone from UIKit to AppKit.

Yay! Runestone now works with... *Checks notes*... UIKit?? 🤨

In order to prepare for AppKit support, I had to rip the internals of Runestone apart and put it together again. For the first time in 36 hours, Runestone now works with UIKit again.

@simonbs congrats! Love Runestone and all your apps so much.
@film_girl Thank you! 🥹