Split now playing, the quiz buttons and song loading out of the quiz

NowPlaying, QuizToolbar and SongControls become their own scenes, the auto
repeat switch moves into the music player scene, and downloading songs moves
into a SongLoader node. The quiz only wires the parts together. Every node
keeps its place on screen.

Screenshots of every UI state are pixel-identical to before, the layout test
still matches the original baseline, and each new part has its own tests.

tests/integration/test_main_window_startup.gd

@@ -28,7 +28,7 @@ func test_next_plays_the_preloaded_song_after_a_slow_song_list() -> void:
 
 
 func test_failed_song_list_is_retried_before_preloading() -> void:
-	_quiz.song_fetch_retry_seconds = 0.05
+	_quiz.song_loader.retry_seconds = 0.05
 	_server.respond_json("/music/list", [], 503)
 	_server.release("/music/list")