Removed WorldTime, and instead introduced the use of CelestialModel

Removed WorldTime, instead CelestialModel should be used (obtained from CelestialSystem) for providing translation of GameTime (obtained from Time) to world time (days, etc).

I had to remove the WorldTimeEvents (ticks), as they heavily depended on some world definition (day length, etc). If someone wants to reintroduce them, they should probably not depend on them anymore, as we could have multiple different world in the multi-verse, each with different day length, or some without days defined at all (no single celestial body).

Also, at the moment a lot of code assumes there is single celestial body, but it's probably too early to eradicate it, and changes to do so, would be substantial and can be done AFTER this change.