The idea is to replace the current OSM provider+plugin by this new code. Differences from old code:
- read-only access - without editing and upload support
- no special provider (using SpatiaLite provider)
- underlying OSM topology accessible from API
- download using Overpass API: fast, customizable, nearly unlimited download
- OSM XML files have to be first imported to a Sqlite3 database, then SpatiaLite layers can be exported
