cc-night-light-page: Add brightness controls

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
7 jobs for nightlight_brightness_controls in 5 minutes and 31 seconds
latest
Status Name Job ID Coverage
  Build
passed build #1247406

00:04:13

 
  Test
passed test #1247407

00:01:18

 
  Manual
manual asan #1247409
asan allowed to fail manual
manual flatpak #1247408
allowed to fail manual
manual msan #1247412
allowed to fail manual
manual tsan #1247410
allowed to fail manual
manual ubsan #1247411
allowed to fail manual