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.

tools/take_screenshots.gd

@@ -33,7 +33,7 @@ func _take() -> void:
 	viewport.gui_embed_subwindows = true
 	viewport.render_target_update_mode = SubViewport.UPDATE_ALWAYS
 	root.add_child(viewport)
-	var main: Control = load("res://MainWindow.tscn").instantiate()
+	var main: Control = load("res://main/MainWindow.tscn").instantiate()
 	viewport.add_child(main)
 
 	var layout: Dictionary = {}