Add player skeleton and playback service

This introduces a dependency on audio_service and implements the
playback service using that package. The UI was adapted to the new
interface.

lib/main.dart

@@ -1,10 +1,13 @@
+import 'package:audio_service/audio_service.dart';
 import 'package:flutter/widgets.dart';
 
 import 'app.dart';
 import 'backend.dart';
 
 void main() {
-  runApp(Backend(
-    child: App(),
+  runApp(AudioServiceWidget(
+    child: Backend(
+      child: App(),
+    ),
   ));
 }