webui: navigation: unique names on entity facet menu items
    
    Fixes issue where it is not possible to define under the same parent:
      { entity: 'bar', facet: 'baz' }
      { entity: 'foo', facet: 'baz' }
    
    Error reporting of invalid menu item names was improved.
    
    Reviewed-By: David Kupka <dkupka@redhat.com>
    Reviewed-By: Thierry Bordaz <tbordaz@redhat.com>