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/Constants.swift

@@ -4,6 +4,8 @@ import SwiftUI
 
 enum Constants {
     static let overlayAnimation = Animation.linear(duration: 0.2)
+    static let aspectRatio16x9 = 16.0 / 9.0
+    static let aspectRatio4x3 = 4.0 / 3.0
 
     static var isAppleTV: Bool {
         #if os(iOS)