Various ThemeDownloader Updates

- `try` added where applicable to ArticleTheme inits
- `ArticleThemePlist` has fixed spelling of theme identifier and conforms to Equatable
- `ArticleTheme` now uses `ArticleThemePlist`
-  `ArticleThemeDownloader` is now a class
- `ArticleThemeDownloader` will now download themes to Application Support/NetNewsWire/Downloads on macOS and iOS.
- `ArticleThemeDownloader` will remove downloaded themes from the Download folder when the application is closed.
- macOS app delegate now observes for theme download fails
- Error display code moved from SceneDelegate to SceneCoordinator so that it can use existing presentError on rootVC.

Mac/Scriptability/AppDelegate+Scriptability.swift

@@ -61,9 +61,9 @@ extension AppDelegate : AppDelegateAppleEvents {
 					}
 					
 					do {
-						try ArticleThemeDownloader.handleFile(at: location)
+						try ArticleThemeDownloader.shared.handleFile(at: location)
 					} catch {
-						print(error)
+						NotificationCenter.default.post(name: .didEndDownloadingThemeWithError, object: nil, userInfo: ["error": error])
 					}
 				}
 				task.resume()