po2idml: Open the template IDML sooner to list translatable files

This will allow to add code in next commit for listing the translatable
files in the IDML package.
This is necessary to be able to group the translated units per translatable
file, and thus build the XPathTree objects for each of them.
po2idml: Call build_unit_tree for each translatable file

This is necessary because the previous code were used for ODF, where each
of the three translatable files had a different root tag, where all the
translatable files in IDML use the same root tag, so the code was unable
to create a separate unit tree for each translatable file within the IDML
package.
COMMENTS

REMOVE WHEN IT WORKS
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.
UNDO   REMOVE THESE COMMENTS