Convert Night Color into a plugin

Night Color adjusts the color temperature based on the current time in your location. It's not a generic color correction module per se.

We need a central component that can be used by both night color and colord integration to tweak gamma ramps and which will be able to resolve conflicts between the two. The Night Color manager cannot be such a thing because of its very specific usecase.

This change converts Night Color into a plugin to prepare some space for such a component.

The tricky part is that the dbus api of Night Color has "ColorCorrect" in its name. I'm afraid we cannot do that much about it without breaking API compatibility.

cc #11