[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