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/integration/test_layout_unchanged.gd

@@ -86,7 +86,7 @@ func before() -> void:
 	viewport.size = Vector2i(1920, 1080)
 	viewport.gui_embed_subwindows = true
 	Engine.get_main_loop().root.add_child(viewport)
-	_main = load("res://MainWindow.tscn").instantiate()
+	_main = load("res://main/MainWindow.tscn").instantiate()
 	viewport.add_child(_main)
 	await UiTour.new(_main).run(func(state: String, popup: Node) -> void:
 		_layout[state] = UiTour.dump_layout(_main, _main if popup == null else popup))