UI: issue critical message if factory cannot resolve an item type
RCORE: allow recursion constraints on XmlNode::xml_string
RCORE: XmlNode: properly handle multiple toplevels as error

* This used to leak all but the last parsed node.
Merge branch 'factory-xmlnode-fixes'

* factory-xmlnode-fixes:
  RCORE: XmlNode: properly handle multiple toplevels as error
  RCORE: allow recursion constraints on XmlNode::xml_string
  UI: issue critical message if factory cannot resolve an item type