core: Split PlaybackManager up into smaller parts

The PlaybackManager was a giant class that handled UI bindings, fetching
stream URLS, playback logic.

It now has been split up into:

- ViewModel::PlaybackManager, which handles UI interfacing and allowing
  to swap out the Model::Playback implementation on the fly.
- Model::PlaybackManager, which is an interface for what a
  PlaybackManager must do, handling queues/playlists, and controlling a
  player.
- Model::LocalPlaybackManager, which is an Model::PlaybackManager
  implementation for playing back Jellyfin media within the application.
- Model::PlaybackReporter, which reports the current playback state to
  the Jellyfin server, for keeping track of played items.
- Model::Player, which handles playing back media from an URL and
  the usual play/pause et cetera.

In a future commit, this would allow for introducing a
Model::RemoteJellyfinPlaybackManager, to control other Jellyfin
instances.

sailfish/qml/harbour-sailfin.qml

@@ -1,6 +1,6 @@
 /*
 Sailfin: a Jellyfin client written using Qt
-Copyright (C) 2020 Chris Josten
+Copyright (C) 2020-2022 Chris Josten
 
 This library is free software; you can redistribute it and/or
 modify it under the terms of the GNU Lesser General Public
@@ -114,7 +114,6 @@ ApplicationWindow {
         id: _playbackManager
         apiClient: appWindow.apiClient
         audioIndex: 0
-        autoOpen: true
     }
 
     Connections {