Fix import of indented lists

Import of indented lists currently does not work, because contentcollector.js does default to bullets in case of an unordered list. This patch makes contentcollector aware of indented unordered lists as well.

I included tests. In the tests I bypass the editor UI and directly call /import and /export URLs
Notices however, that they currently fail because there is another bug in the list-parsing code. I will issue another pull request soon including the patch for this other bug and they will merge without conflicts because they do not touch the files in this request.