Prevent warning when offline

- Update check creates a warning
    GUI [WARN ] Couldn't check for version update.
  followed by
    java.net.SocketException
  and it's stacktrace.
  In case of being offline, or when using MasterPassword behind a
  restrictive firewall/proxy, this can be disturbing and can be avoided
  by using an option to prevent update check.
  Use "-Doffline" or "-Doffline=true" in GUI startup command to activate
  contidional update check.