Implement in-game cron engine

[lstrzebinczyk]

Theres a lot of thing that I want to happen only once in a while. Those are things like

• update vegetation, once daily or weekly,
• update roads, once daly or weekly
• update day/night mask, hourly
• update characters, once per 2 minutes?

For that I need to implement cronlike system. I don't need to check all of the events that are supposed to happen, only the next one.

Reads: Franta-Maly event list