Add Location Markers (Part 1)

[AdenKoperczak]

A minimum viable implementation of location markers. The location markers are on a new map layer. They can be configured from the location marker manager. They are saved to location-markers.json in the Appdata directory.

There are several more features that I would like to add, but I want to make sure I have a good starting point before I build this out more. Dan, feel free to review this as you have time.