Issue #1714462 by n3or, sun, Gábor Hojtsy: Convert language negotiation settings to configuration system.

PR for https://github.com/backdrop/backdrop-issues/issues/170
One more thing, it looks like in language_negotiation_configure_session_form(), system_settings_form() may still be getting used. Convert this to the typical $form['actions']['submit'] button. And in this new submit handler, add a backdrop_set_message() to let the user know that the action was saved it can be a simple:
```
  backdrop_set_message(t('Configuration saved.'));
```