Toggle Map Pins

I tend to use many map pins for planning where to put districts and wonders and things get very cluttered and some elements are obscured by the pins.
It would be great if there was a way to toggle map pins on and off like you can for yields and resource icons

No files, tests, or entry points are named. Start by locating the existing UI controls for toggling yields and resource icons, then trace how map pins are rendered; done means a control can hide and restore map pins without affecting other map elements.

Written by the indexing model from the issue text.