Path query can be relative, absolute from navigation root and absolute from site root

We need to distinguish between absolute path to site root vs. absolute path from navigation root.

For vanilla plone this makes no difference, but as soon as plone.app.multilingual or collective.lineage is installed (or any other addon introducing custom INavigationRoot) we need to be able to search from portal root explicitely.

Example usecase is a plone.app.multilingual installation, where each language folder should contain a collection with data displayed from portal_root/foo, which lives next to language folders.

Also relativePath is adopted to break parent lookup ('../') at IPloneSiteRoot rather than INavigationRoot

cheers
major housekeeping should be done in a separate PR