display: Add Night Light color slider

This adds a scale to change the color temperature from 3000K to
6000K. A mark is added to the default value and a second one for
aesthetics.

Initial implementation by Benjamin Berg
Color choices by Daniel Foré and elementary OS

Closes #147
11 jobs from nightlight-temperature in 3 minutes and 46 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #165708
build

00:02:14

 
  Test
passed #165710
asan

00:01:31

passed #165709
test

00:01:17

 
  Manual
manual #165715
allowed to fail manual
armv8
manual #165711
allowed to fail manual
flatpak
manual #165714
allowed to fail manual
msan
manual #165716
allowed to fail manual
ppc64le
manual #165712
allowed to fail manual
tsan
manual #165713
allowed to fail manual
ubsan
 
  Review
manual #165717
allowed to fail manual
review
manual #165718
allowed to fail manual
stop_review