Send message when hovering islands

To be used for fertility display (even for islands without settlement).
Unify cityinfo hide somewhat

Always add the short delay, not only when moving away from local_player
settlements. Should be easier to hook into this way (re:fertility).
Coding style
Remove fertility display from cityinfo

This will be an own widget soon.
FertilityInfo widget to complement CityInfo

Mostly yummy copy/pasta. I could not bear copying over the positioning
code however, so that one is coupled to cityinfo for the time being.

Needs an own widget mostly because it's also supposed to show up when
the island in question has not been settled on yet.