Reimplement UI using SwiftUI

[jakebromberg]

Apple introduced SwiftUI at WWDC this year. The framework is still developing and is subject to drastic API changes from beta to beta. It's also iOS 13 only. I don't mind dropping support for all versions but the latest release, but I expect the framework to continue to evolve and stay plagued with performance problems throughout the year. I wouldn't suggest shipping a SwiftUI app until next year, but we can play around with the framework for now.