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.

core/include/JellyfinQt/platform/freedesktop/mediaplayer2player.h

@@ -16,6 +16,8 @@
 #include <QtCore/QObject>
 #include <QtDBus/QtDBus>
 #include <QMediaPlayer>
+#include "JellyfinQt/model/player.h"
+
 QT_BEGIN_NAMESPACE
 class QByteArray;
 template<class T> class QList;
@@ -190,9 +192,9 @@ private:
     ViewModel::PlatformMediaControl *m_mediaControl;
     void notifyPropertiesChanged(QStringList properties);
 private slots:
-    void onCurrentItemChanged(ViewModel::Item *newItem);
-    void onPlaybackStateChanged(QMediaPlayer::State state);
-    void onMediaStatusChanged(QMediaPlayer::MediaStatus status);
+    void onCurrentItemChanged();
+    void onPlaybackStateChanged(Jellyfin::Model::PlayerStateClass::Value state);
+    void onMediaStatusChanged(Jellyfin::Model::MediaStatusClass::Value status);
     void onPositionChanged(qint64 position);
     void onSeekableChanged(bool seekable);
     void onPlaybackManagerChanged(ViewModel::PlaybackManager *newPlaybackManager);