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_stats_after_win.gd

@@ -24,7 +24,7 @@ func test_winning_turns_statistics_off() -> void:
 func test_resetting_points_turns_statistics_back_on() -> void:
 	await _give_point_with_keybind(1)
 
-	_press("ResetPointsButton")
+	_press("QuizToolbar/ResetPointsButton")
 
 	assert_bool(Settings.add_to_stats).is_true()
 
@@ -33,7 +33,7 @@ func test_statistics_turned_off_by_hand_stay_off_after_reset() -> void:
 	Settings.add_to_stats = false
 	await _give_point_with_keybind(1)
 
-	_press("ResetPointsButton")
+	_press("QuizToolbar/ResetPointsButton")
 
 	assert_bool(Settings.add_to_stats).is_false()
 
@@ -41,7 +41,7 @@ func test_statistics_turned_off_by_hand_stay_off_after_reset() -> void:
 func test_settings_checkbox_shows_statistics_off_after_win() -> void:
 	await _give_point_with_keybind(1)
 
-	_press("SettingsButton")
+	_press("QuizToolbar/SettingsButton")
 
 	assert_bool(_stats_checkbox().button_pressed).is_false()
 
@@ -50,13 +50,13 @@ func test_songs_after_a_win_are_not_counted() -> void:
 	await _give_point_with_keybind(1)
 	var played_before: int = _server.count("/music/played?song=0")
 
-	_press("ShowAnswerButton")
+	_press("SongControls/ShowAnswerButton")
 
 	assert_int(_server.count("/music/played?song=0")).is_equal(played_before)
 
 
 func test_winning_score_cannot_go_below_one() -> void:
-	_press("SettingsButton")
+	_press("QuizToolbar/SettingsButton")
 	var lower_button: Button = _quiz.get_node("SettingsWindow").get_node("%LowerButton")
 
 	for i: int in 3: