• Add support for Swift Package Manager and associated file restructuring
• Fix broken unit tests that were not migrated from UIWebView to WKWebView
• Fix assorted warnings

Fix a long-standing bug which prevented navigation to YouTube when tapping some player UI elements.

Fix bug introduced by e9dbe9e regarding mapping of player state. The value returned from the player has a lower-bound of "-1", whereas YTPlayerState is a 0-based NSInteger enum.

Fix UI scaling in the WebView

Migrate to WKWebView and other general clean-up and modernization.