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_list_panel.gd

@@ -17,7 +17,7 @@ func before_test() -> void:
 		song.song_title = "Song %d" % i
 		song.has_played = true
 		Playlist.add_to_playlist(song)
-	_runner = scene_runner("res://MusicListPanel.tscn")
+	_runner = scene_runner("res://quiz/MusicListPanel.tscn")
 	_panel = _runner.scene()