Improved Captions handling

New options for captions in `Settings-Player`:

- Always show captions
- Default language

User can now select whether they want to show captions automatically when the video starts, and select the language.

Captions selector now shows proper name -> `English (en)` instead of only `en`

Model/Player/Backends/MPVClient.swift

@@ -405,6 +405,14 @@ final class MPVClient: ObservableObject {
         setString("video", "no")
     }
 
+    func setSubToAuto() {
+        setString("sub", "auto")
+    }
+
+    func setSubToNo() {
+        setString("sub", "no")
+    }
+
     var tracksCount: Int {
         Int(getString("track-list/count") ?? "-1") ?? -1
     }