Sidewalk overlay: Allow editing the kind of footway

[westnordost]

The sidewalk overlay displays footways and paths, currently only for orientation and to see whether a sidewalk has been mapped separately. The footway key allows to define the kind of footway. It should be possible to set the kind of footway in the overlay.

The UI should look similar to the Lit way or separate cycleway UI. Selection options should be at least sidewalk, crossing, traffic_island, nothing (i.e. tag not set or unsupported), access_aisle and link.

footway is used first and foremost to differentiate (other) foot paths from sidewalks & crossings. It is not possible to create a quest to have users define the kind of footway because there is no catch-all "other footways" footway=* value (maybe footway=path but it is not documented). Overlays however do not have this constraint, so it would be possible to specify the kind of footway within the sidewalk overlay, and it seems to be a good fit.

The information whether a footway is really a separately mapped sidewalk plus sidewalk:both=separate helps software to properly associate each sidewalk with a street. Also, knowing which footways are sidewalks, e.g. rendering software could opt to not show or show sidewalks differently from other footways etc.