idml2po: Completely rework IDML extraction

This correctly converts placeables to X and G tags. It also skips
unparseable tags.

This breaks the ODF tests because part of the code is adjusted to IDML
needs without trying to keep compatibility with ODF. This tests breaking
is fixed in a later commit.
imdl2po: 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>
idml2po: Output correct PO
idml2po: Avoid having repeated ids on the resulting output
po2idml: Adjust roundtrip to handle PO units having XLIFF placeables

Note that this still doesn't output the processing instructions OK.
po2idml: Keep processing instructions unchanged

This is done by avoid setting a new text for processing instructions.
IDML: Get ODF tests working again
--------------WORKING:   ADD IDML roundtrip tests
COMMENTS

REMOVE WHEN IT WORKS