First radio browser screens

2 screens for the new radio browser functionality in Rigelian are nearing completion. First the main screen with a list of popular and highest rated stations, and second the search screen.

The implementation of these screens is based on various new capabilities that were introduced in iOS 13: compositional layout for UICollectionView and diffable datasources. While there is some learning curve to this, the result is a much smoother user experience, and less code that is easier to maintain.