[colorcorrection] Add "Constant" mode

Summary:
Currently, the Night Color manager supports three operation modes:

* Automatic: in this mode, screen color temperature is computed based on
the current position of the Sun. In order to calculate sunrise & sunset
times, the manager needs coordinates of the user, which are provided by
Plasma;

* Location: this mode is very similar to the Automatic, except one minor
detail: user needs to provide his/her/their location. This mode can be
very useful if coordinates provided by Plasma are incorrect;

* Timings: unfortunately we can't compute timings of the Sun for people
living near Earth poles. This mode allows the user to specify timings of
sunrise and sunset as well the transition time.

This change introduces another mode, called Constant. With this mode the
screen color temperature is constant throughout the day. The new mode
can be useful for people wishing constant screen color temperature or
just for people living near Earth's North or South poles.

Reviewe...