Navigation portlet doesn't recurse

[glemignot]

The navigation portlet in Plone 5 never displays more than the elements in the current level, it doesn't even recurse even if the UI says it should recurse (depending on the parameters).

Here is a partial patch, that re-enables the feature if a non-zero depth is entered on the portlet, but it doesn't work (due to ExtendedPathIndex) if the depth is 0, so the UI should probably be updated too to indicate that a depth of 0 means no recursion.

navigation.diff.txt