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.

tests/fakes/main_window_test_suite.gd

@@ -38,7 +38,7 @@ func before_test() -> void:
 	_server.respond_bytes("/music/played?song=0", PackedByteArray())
 
 	_before_window_starts()
-	_runner = scene_runner("res://MainWindow.tscn")
+	_runner = scene_runner("res://main/MainWindow.tscn")
 	_window = _runner.scene()
 	_quiz = _window.get_node("Quiz")
 	# The debug build lowers the winning score on startup; keep it out of reach