draft: extend night light to change the color temperature to the desired value, instead of assuming the display to be 6500K

Draft because we need to expose this somewhere in the UI. I think we should

• add a "daylight" color temperature slider in the display settings, which set the same setting as the daylight slider in the night light KCM
• remove "always on night light" as an option in the night light settings, as it's redundant now, and allow the user to modify the daylight color temperature instead
• extend the range of the night light slider to go above 6500K maybe? Could be done separately