UI: Re-write GUI using SwiftUI

[russellhancox]

The existing Santa UI is very basic and uses 'old-fashioned' XIBs for layout etc. The UI hasn't been modified in quite a long time and any changes now would be difficult to review. We're also as a team not using Xcode anymore so requiring interface builder for modifications is suboptimal too.

SwiftUI is the future of UI-work on Apple platforms and has very nice syntax for programmatically creating views. While we generally have avoided Swift (for good reasons), this limited use is easy to integrate, particularly thanks to the good interop with existing ObjC code.