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.

project.godot

@@ -16,16 +16,16 @@ compatibility/default_parent_skeleton_in_mesh_instance_3d=true
 
 config/name="MusicPlayer"
 config/version="1.9.0"
-run/main_scene="res://MainWindow.tscn"
+run/main_scene="res://main/MainWindow.tscn"
 config/features=PackedStringArray("4.7", "GL Compatibility")
 config/icon="res://icon.svg"
 
 [autoload]
 
-GameLog="*res://game_log.gd"
-Settings="*res://settings.gd"
-Playlist="*res://playlist.gd"
-MusicServerClient="*res://music_server_client.gd"
+GameLog="*res://common/game_log.gd"
+Settings="*res://data/settings.gd"
+Playlist="*res://data/playlist.gd"
+MusicServerClient="*res://server/music_server_client.gd"
 
 [debug]