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.

quiz_toolbar.gd

@@ -0,0 +1,25 @@
+class_name QuizToolbar
+extends Control
+## The row of quiz buttons under the top bar: reset playlist, reset points, settings
+## and sound test.
+
+signal reset_playlist_pressed
+signal reset_points_pressed
+signal settings_pressed
+signal sound_test_pressed
+
+@onready var reset_playlist_button: Button = $ResetPlaylistButton
+@onready var sound_test_button: Button = $SoundTestButton
+
+
+func _ready() -> void:
+	reset_playlist_button.pressed.connect(reset_playlist_pressed.emit)
+	$ResetPointsButton.pressed.connect(reset_points_pressed.emit)
+	$SettingsButton.pressed.connect(settings_pressed.emit)
+	sound_test_button.pressed.connect(sound_test_pressed.emit)
+
+
+## Disables the buttons that use the song list, e.g. while the server syncs it.
+func set_locked(locked: bool) -> void:
+	reset_playlist_button.disabled = locked
+	sound_test_button.disabled = locked