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/apimodel.h

@@ -489,6 +489,10 @@ public:
         this->endResetModel();
     }
 
+    const QList<QSharedPointer<T>> &toList() {
+        return m_array;
+    }
+
     // From AbstractListModel, gets implemented in ApiModel<T>
     //virtual QHash<int, QByteArray> roleNames() const override = 0;
     /*virtual QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override = 0;*/