Lift requirement on values of hippo:paths property that the nodes they reference are of mix:referenceable

Currently an ancestor node must be of type mix:referenceable in order to be added to the hippo:paths property. This is not necessary because the hippo:paths property is of type String, not of type Reference, and is probably legacy from JCR 1.0.

As is stands now, this requirement only represents a risk: when the mix:referenceable mixin of an ancestor is removed accidentally (for instance from the /content node) this may result with queries returning inadequate results.