include borg.localrole-derived role assignments on sharing tab

This makes local role assignments obtained from borg.localrole adapters show up on the Sharing tab as inherited roles.

The work is adapted from @datakurre's code: http://webcache.googleusercontent.com/search?q=cache:18h4krMP8_UJ:lists.plone.org/pipermail/plone-product-developers/2012-December/011958.html+&cd=1&hl=en&ct=clnk&gl=us

If this looks acceptable we need to merge it to master too, which I can help with.