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.

common/game_log.gd

@@ -0,0 +1,21 @@
+extends Node
+## Writes messages about what the game is doing, for finding problems.
+##
+## Registered as the [code]GameLog[/code] autoload. Use [method debug] instead of
+## [code]print()[/code]: the messages show in debug builds and the editor, and are left
+## out of released games. Problems the player may notice go through [method warning].
+
+
+## Prints [param parts] joined together, like [code]print()[/code], in debug builds only.
+func debug(...parts: Array) -> void:
+	if OS.is_debug_build():
+		print(_join(parts))
+
+
+## Reports a problem in the editor's debugger and the console, in every build.
+func warning(...parts: Array) -> void:
+	push_warning(_join(parts))
+
+
+func _join(parts: Array) -> String:
+	return "".join(parts.map(func(part: Variant) -> String: return str(part)))