- fix bug where a property tree saved with writeProperties() and read back

- fix bug where a property tree saved with writeProperties() and read back
  in with readProperties() would not look the same, because element indices
  of '0' were even dropped when a node has a "secret" value *and* children

- introduce "omit-node" modifier attribute for the "include" attribute.
  This inserts the given file in place of the including node, while the
  node is dropped. This is desirable for multiple includes (which can't
  be done by multiply using the "include" attribute, as this isn't valid
  XML spec syntax)