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_players_panel.gd

@@ -11,7 +11,7 @@ func before_test() -> void:
 	_snapshot = AutoloadSnapshot.take([Settings])
 	Settings.player_array.clear()
 	Settings.edit_players = false
-	_runner = scene_runner("res://PlayersPanel.tscn")
+	_runner = scene_runner("res://quiz/PlayersPanel.tscn")
 	_panel = _runner.scene()