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>

Shared/Player/Controls/PlayerControls.swift

@@ -265,7 +265,7 @@ struct PlayerControls: View {
 
     var controlsBackgroundURL: URL? {
         if let video = player.videoForDisplay,
-           let url = thumbnails.best(video)
+           let url = thumbnails.best(video).url
         {
             return url
         }