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_sync_popup.gd

@@ -12,7 +12,7 @@ func before_test() -> void:
 	Settings.currently_syncing = false
 	_server = FakeHttpTransport.new().install()
 	_server.respond_bytes("/sync", PackedByteArray())
-	_runner = scene_runner("res://SyncPopup.tscn")
+	_runner = scene_runner("res://popups/SyncPopup.tscn")
 	_window = _runner.scene()