Section-Filter in topic view (Subsite)

This PR adds section-support for the topic view.
It adds a filter per Section and displays only the contents of one section at a time.

- A section is by default any object providing "INavigationRoot". Since the Subsite (ftw.subsite) provides this interface, the subsite are automatically a section.
- The Plone site is always a section.
- When there is only one section, the filter is not displayed. So when not having a complex setup with ftw.subsite or other section-implementations, no such section things are displayed and it just works.
- When having multiple "similar" trees in different sections, the back-references are merged and re-grouped by section, so that the topic view also shows references from other topics. The similarity detection is path based and requires to have the exact same path to the first parent section (`INavigationRoot`).

/cc @maethu