Indicate when night light is unavailable
It's my understanding that, if someone's running Wayland with the NVIDIA driver, then night light won't work. It would be good to reflect this state in the settings UI.
Tentative mockup:
![Screenshot_from_2022-02-22_11-40-01](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
@jadahl @ofourdan @csoriano