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_music_player_scene.gd

@@ -9,7 +9,7 @@ var _player: MusicPlayer
 func before_test() -> void:
 	_snapshot = AutoloadSnapshot.take([Settings])
 	Settings.auto_repeat_song = true
-	_runner = scene_runner("res://MusicPlayer.tscn")
+	_runner = scene_runner("res://player/MusicPlayer.tscn")
 	_player = _runner.scene()