From GPX store the origin Track|Route|Waypoint

It would be a great addition if we could add the origin from where a feature was extracted.

For example if you have GPX file with a track and a route, you will receive two features, but you don't know which feature was the track and which feature was the route.

Waypoints are obviously `type: 'Point'`, so that would be straight forward. But at least for route and track it would be handy if one could determine which is which.

I could provide a PR for this adding a field to the properties like `origin: wpt|rte|trk` (or something along these lines).

Start by tracing the GPX conversion path that creates GeoJSON features for tracks, routes, and waypoints. Confirm how feature properties are assembled, then define completion as each extracted feature identifying its GPX origin with a consistent value such as wpt, rte, or trk.

Written by the indexing model from the issue text.