Migrate OSM editing to use JSON instead of XML
Since some time (see changelog in https://wiki.openstreetmap.org/wiki/API_v0.6) the OSM API (even though it's still v0.6) supports JSON.
This should mean we could skip the glue C code implementing parsing and serializing objects into the XML format (this is needed as libxml2 doesn't have GObject introspection, so it can't be used directly from GJS).
Assuming the JSON parts now covers everything we need, it should be possible to implement the OSM* classes from C library in JS and use the JSON format for parsing and serializing.