Hide "Update library" menu item if the current backend is not connectable

The "Update library" menu item should not be offered if the current backend is not connectable since it cannot be contacted to initiate a library update. Doing so only results in the error message "Exception caught while calling XBMC API: Unable to connect to [HOST] . Error #0: stream_socket_client(): unable to connect to [HOST]&#32;(Connection refused) (0)".