UI improvements, player state refactor

Model/SingleAssetStream.swift

@@ -0,0 +1,12 @@
+import AVFoundation
+import Foundation
+
+final class SingleAssetStream: Stream {
+    var avAsset: AVURLAsset
+
+    init(avAsset: AVURLAsset, resolution: Resolution, kind: Kind, encoding: String) {
+        self.avAsset = avAsset
+
+        super.init(audioAsset: avAsset, videoAsset: avAsset, resolution: resolution, kind: kind, encoding: encoding)
+    }
+}