Make respecting the navroot for searches configurable.

By default, `plone.app.search` always constrains searches to the navigation root by adding a `path` filter to the query in `SearchView.filter_query()`.

This change overrides `filter_query()` in order to make this behaviour configurable. Only if `ISearchSettings.respect_navroot` is set (defaults to False), searches will be constrained to the navigation root.

Note: This requires the ftw.solr profile to be upgraded to 1003.
@buchi: Since I released ftw.solr since we discussed this, I decided to create a 1003 upgrade profile + upgrade step anyway.

/cc @maethu @jone