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.

themes/default_theme.tres

@@ -0,0 +1,6 @@
+[gd_resource type="Theme" load_steps=2 format=3 uid="uid://8bllart6xn1f"]
+
+[ext_resource type="StyleBox" uid="uid://bsv6yb0yir04l" path="res://themes/style_box_flat.tres" id="1_uierc"]
+
+[resource]
+PanelContainer/styles/panel = ExtResource("1_uierc")