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/CMakeLists.txt

@@ -15,6 +15,9 @@ include(GeneratedSources.cmake)
 set(JellyfinQt_SOURCES
         src/model/deviceprofile.cpp
         src/model/item.cpp
+        src/model/player.cpp
+        src/model/playbackmanager.cpp
+        src/model/playbackreporter.cpp
         src/model/playlist.cpp
         src/model/shuffle.cpp
         src/model/user.cpp
@@ -48,6 +51,9 @@ list(APPEND JellyfinQt_SOURCES ${openapi_SOURCES})
 set(JellyfinQt_HEADERS
         include/JellyfinQt/model/deviceprofile.h
         include/JellyfinQt/model/item.h
+        include/JellyfinQt/model/player.h
+        include/JellyfinQt/model/playbackmanager.h
+        include/JellyfinQt/model/playbackreporter.h
         include/JellyfinQt/model/playlist.h
         include/JellyfinQt/model/shuffle.h
         include/JellyfinQt/model/user.h