Show in the top bar when a newer version is available

At start and after switching server, the game asks the server for the newest
MusicPlayer release and shows "Version X is available" in the top bar when it
is newer than this one.

main/main_window.gd

@@ -38,6 +38,7 @@ func _ready() -> void:
 	sync_popup.sync_started.connect(_on_sync_started)
 	sync_popup.sync_finished.connect(_on_sync_finished)
 	quiz.server_changed.connect(search_popup.get_list_of_soundtracks)
+	quiz.server_changed.connect(top_bar.check_for_update)
 	quiz.event_logged.connect(log_popup.add_log_row)
 	statistics_view.soundtrack_selected.connect(soundtrack_stats_popup.show_soundtrack)