locator: Rework the location updating logic

Previously, we would emit the LocationUpdated signal for any location
property change, disregarding the DistanceThreshold value. A change in
the "description" property would trigger the LocationUpdated signal,
even though we were well below the threshold.

This commit changes it to emit LocationUpdated only when we cross the
threshold. We keep track of the previous location where LocationUpdated
was last emitted. While below the threshold, we update the location
properties, but don't notify the clients; the clients only get notified
once the threshold is crossed.

https://bugs.freedesktop.org/show_bug.cgi?id=69105
interface: Add some documentation

Document the DistanceThreshold property and LocationUpdated signal.

https://bugs.freedesktop.org/show_bug.cgi?id=69105