widgets: support sorting choices by proximity when choices map to geographical locations

[tahini]

As a followup of #1603, some questions have values that map to geographical location, like bus/train stations, etc. Knowing the closest location in the trip, those choices should be sorted by proximity to a specific place.

Widgets that support choices could have an additional property added to choices that would contain the matching geographical location and an additional widget property could get the location to compare to.