Add a Sensor for maintaining paths over time. #60
Description
This is a similar feature to #59, but it's slightly more efficient. If you constantly need a path, we should just update a little sensor thing the same way we update an agent.
An example of this is Half-Life's "smell AI", which tracks the distance of the player from an enemy along the navmesh. This means you could be right next to your enemy, but if there's a wall between you two, the enemy couldn't smell you.