Add UI controls for setting dim and default brightness.

Necessary for new feature that may be added to gnome-settings-daemon

Using 2 spin buttons instead of a single slider because
while the slider allowed for setting dim brightness level,
I couldn't figure out a way to revert to the brightness before
nightlight was enabled.

By allowing the user to set the default brightness level,
it becomes easier to reset the brightness when night light is off.

More information about the feature can be found in the GSD MR

GSD MR: gnome-settings-daemon!230

Closes GSD Issue: gnome-settings-daemon#528