Sort the scripts and scenes into folders

main/ holds the main window and top bar, quiz/ the quiz and its parts,
player/ the music player, popups/ every popup and what they share, data/ the
songs, players, playlist and settings, server/ the MusicServer client,
common/ the list row and GameLog, and themes/ the themes and style boxes.

player/volume_slider.gd

@@ -0,0 +1,24 @@
+class_name VolumeSlider
+extends HSlider
+## A slider that sets the volume of an audio bus.
+
+## The audio bus whose volume the slider sets.
+
+@export var audio_bus_name: String = "music"
+
+var _bus: int
+
+
+func _ready() -> void:
+	_bus = AudioServer.get_bus_index(audio_bus_name)
+	value_changed.connect(_on_value_changed)
+	value = db_to_linear(AudioServer.get_bus_volume_db(_bus))
+
+
+## Sets the bus volume to [param changed_value], from 0 (silent) to 1 (full).
+func change_volume(changed_value: float) -> void:
+	AudioServer.set_bus_volume_db(_bus, linear_to_db(changed_value))
+
+
+func _on_value_changed(changed_value: float) -> void:
+	AudioServer.set_bus_volume_db(_bus, linear_to_db(changed_value))