[WIP] Switch to xctool

**:warning: NOT READY TO MERGE :warning:**

Use [xctool](https://github.com/facebook/xctool) instead of `xcodebuild`, because dealing with the latter requires way too many hacks. Note that this means that `xctool` _must_ be installed on the build machine.

I also took this opportunity to significantly clean up the code. It actually looks kinda nice, if you close one eye and view it from a distance.