Improved thumbnail handling

- ThumbnailsModel optionally returns the quality
- Have constants for 4:3 and 16:9 aspect ratio
- Add high quality options for thumbnails
- Rename Highest quality to Best quality
- make 4:3 thumbnails fill the VideoCell
- use .maxes instead of .maxresdefault (the latter sometimes returns white images)

Signed-off-by: Toni Förster <toni.foerster@gmail.com>

Model/ThumbnailsModel.swift

@@ -20,21 +20,23 @@ final class ThumbnailsModel: ObservableObject {
         return unloadable.contains(url)
     }
 
-    func best(_ video: Video) -> URL? {
+    func best(_ video: Video) -> (url: URL?, quality: Thumbnail.Quality?) {
         for quality in availableQualitites {
             let url = video.thumbnailURL(quality: quality)
             if !isUnloadable(url) {
-                return url
+                return (url, quality)
             }
         }
 
-        return nil
+        return (nil, nil)
     }
 
     private var availableQualitites: [Thumbnail.Quality] {
         switch Defaults[.thumbnailsQuality] {
         case .highest:
-            return [.maxresdefault, .medium, .default]
+            return [.maxres, .high, .medium, .default]
+        case .high:
+            return [.high, .medium, .default]
         case .medium:
             return [.medium, .default]
         case .low: