po2idml: Correctly handle tags without namespace
po2idml: Export CDATA as is
po2idml: Output standalone="yes" on XML declaration
po2idml: Add function to retrieve DOM objects for pounit source and target

Since we want to reuse as much as possible the previous code for ODF, it
is necessary to provide DOM objects for pounit source and target, like
the ones present in xliffunit.source_dom and xliffunit.target_dom.

This change doesn't affect the ODF converters.
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