--------------WORKING:   ADD IDML roundtrip tests
WORKING ON IT:     Now X and G tags work!!!!

This skips unparseable tags.
Properly extract Content tags with childs before the text

This applies to Content tags with any child before the text, like for
example processing instructions, or that don't have any text outside any
of its children. Examples:

<?ACE 7 ?>Some text
<tag>Some text</tag>
ALMOST: idml2po outputs correct PO
idml2po: Avoid having repeated ids on the resulting output
COMMENTS
ROUNDTRIP WORKING: still not outputting the processing instructions OK
IDML: Avoid setting a new text for processing instructions

This is necessary if we want to keep the processing instructions unchanged
we performing the roundtrip.