place: Add osm-id property

We can end up with places that have the same name when we search
using geocode-forward. It would help to be able to keep them apart.
Having the OpenStreetMap id as a property will also make it possible
to reference the place to OpenStreetMap dbs for editing purpopses.

https://bugzilla.gnome.org/show_bug.cgi?id=726367