OMFOREIGN support in default.ntn

[clange]

At the moment, OMFOREIGN is not supported properly.

I don't exactly know how a proper rendering of OMFOREIGN should look, but I think it should simply be a copy of the child nodes. For example, whenever an application expects OpenMath? but gets something (XML or text) that is not OpenMath?, it can report an error like this:

<OME>
  <OMS cd="error" name="unexpected_..."/> <!-- the "error" symbol -->
  <OMFOREIGN>
    <!-- the unexpected content -->
  </OMFOREIGN>
</OME>

Particularly in the case of unexpected XML, OMFOREIGN must be used, as non-OpenMath? XML is not allowed inside OME. A practical scenario where this is needed is e.g. error handling in the integration of the formula editor with SWiM.

[kohlhase]

I think that the OMFOREIGN should not be presented at all. If we want to present it (e.g. in an error) then it should be presented verbatim as pre-formatted CDATA section.