Notify players when finding city

[jdrueckert]

When finding a city, sometimes buildings take a while to be generated so the only indicator of having found a city is the city's name floating high above the player. This can easily be overlooked, so one idea would be to add a notification that pops up when a player finds a city.
For the time being, I can even imagine that it would be interesting for other players as well if a player found a city, so this information could be broadcast and the notification be created for every client.

For the notifications I'd recommend using the Notifications module.
For finding a suitable event to cause the notification to show up, I'd suggest looking in the DynamicCities and Cities modules for the respective code path or event that indicates the player having found a city. If there's no event yet, we should consider creating and using one.