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.

qtquick/qml/main.qml

@@ -110,7 +110,7 @@ ApplicationWindow {
                 enabled: playbackManager.hasPrevious
             }
             Button {
-                readonly property bool _playing: playbackManager.playbackState === MediaPlayer.PlayingState;
+                readonly property bool _playing: playbackManager.playbackState === PlayerState.Playing
                 anchors.verticalCenter: parent.verticalCenter
                 text:  _playing ? "Pause" : "Play"
                 onClicked: _playing ? playbackManager.pause() : playbackManager.play()