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/VolumeSlider.tscn

@@ -0,0 +1,10 @@
+[gd_scene load_steps=2 format=3 uid="uid://w400rnew7453"]
+
+[ext_resource type="Script" uid="uid://bgviu7smansj1" path="res://player/volume_slider.gd" id="1_1aufb"]
+
+[node name="VolumeSlider" type="HSlider"]
+offset_right = 8.0
+offset_bottom = 16.0
+max_value = 1.0
+step = 0.001
+script = ExtResource("1_1aufb")