#398: Multiple Repositories - don't pull yet

At the moment, this PR is at the let's-have-a-second-opinion-on-ideas level. Please don't pull yet.

Things to do:
- [ ] really add / remove  repositories
- [ ] store repositories in the json file
- [ ] possibly ordering of repositories (when multiple ones provide the same CKAN)
- [ ] adapt the GUI to multiple repositories
- [ ] and many other things I forgot :-)