Show visible locations

[MattEqualsCoder]

Most tracker offer the ability to see what locations you can peak at. We should probably do that too.

I think this would be pretty simple. We could add something similar to the canAccess delegate, but it would default to false. For the map, I think we could probably mark it as a blue triangle or something to be different from the other location icons. Not sure how we would represent this in the location list.