Use real URL, take care of default pages

If I want do share something like "/events", I do not want to have the URL to have as "events/aggregator". This is important for all folder that have a "default page".

see https://svn.plone.org/svn/plone/plone.app.layout/trunk/plone/app/layout/globals/interfaces.py for the various methods to get the URL of an object