Separate celestial events into new package

This is the last part of my refactoring of WorldTime. It adds `CelestialSystem`, an interface for solar events that also controls the position of the sun for the sky sphere. Noteworthy changes:

* The system is not an `UpdateSubscriberSystem` as it needs to be known to `SkySphere` and I wanted to explicitly control which single implemention is used.

* It is initialized as part of `InitialiseWorld` as it seems to be the most reasonable place to me.

* It is registered at the `ComponentSystemManager` manually

* Dusk/Dawn/Midday/Midnight are now part of these sun-related events