Reuse UserLocationMarker when location updates

Instead of creating a new UserLocationMarker whenever a location update is received from GeoClue, create one UserLocationMarker and reuse it. This involves changes to MapMarker so that it will update its location when the underlying Place's location changes.

This prevents the marker's bubble from flickering when the location changes, and it should make it easier to implement a follow-location mode. Also fixes #202 (closed).