[RFC][Patch] Extend opendata plugin to support CKAN open data platforms

[Woazboat]

Extend the opendata plugin to be able to fetch resources from CKAN open data platforms.

​http://docs.ckan.org/en/2.9/user-guide.html#what-is-ckan

• New 'jackan' CKAN client plugin (​https://github.com/opendatatrentino/jackan)
• New CKAN data set handler that fetches resources via CKAN API
• Refactor opendata menu entry creation to allow modules to provide special menu item handling
• GUI to allow users to search and download datasets from CKAN open data platforms from within josm (can be used in modules for chosen open data platforms)

Adds a module for ​https://www.data.gv.at/

• Dataset search interface
• Some sample CKAN data set handlers with special data postprocessing included

[taylor.smock]

@Woazboat: I know this is kind of old, but a few comments:

1. I'd strongly prefer to avoid having to manually add CKAN sources in the future. In the MapWithAI plugin, I've got a JSON that is requested and parsed for sources.
2. It seems like this is more for an import or import-like process. I'm not certain if we want to have processing code for a specific import in a plugin. This is something that really should be done on the dataset before serving it to OSM editors.
3. Have you considered running an osm server or osm-like server (e.g. ​https://gitlab.com/smocktaylor/serve_osm_files -- I really need to make this easier to configure for specific use cases ) and opening a PR for ​https://gitlab.com/gokaart/JOSM_MapWithAI/-/blob/pages/public/json/sources.json ? Alternatively, you can try going through Esri (which would also make it available in RapiD).