Virtual navigation: check the number of vocabularies in the case of a hierarchical one, add a warning if needed

If the first vocabulary is hierarchical, then we need to check that at least another vocabulary is provided, otherwise display a warning.
We can even go further, and check that:

• If there are more than 1 vocabularies provided, then the second one must be:
  • Either a Hierarchical vocabulary and in this case equals to the first one (case of a unique Hierarchical vocabulary that holds both root and child entries)
  • Either a Child vocabulary with the first vocabulary as a parent.
• Maybe we should also not suggest the Child vocabularies in the first dropdown list, since it does not make sense.