[ANN] ODF extension schema & validation

hi all,

the result of a TDF-sponsored project have landed on master now: we have 
an ODF schema that contains LibreOffice extensions, and this is used in 
all unit tests if --with-export-validation is given, which is the default.

notably this means that if you add a new feature to the ODF filters and 
you add the required unit test for the new feature, then most likely the 
test will fail with a complaint from the validator; in this case the 
schema needs to be updated to contain the new elements and attributes.

the schema files are in core.git in the schema/libreoffice directory.

the extension schema uses the RelaxNG "include" feature to refer to the 
ODF schema; this means that it only contains those parts of the schema 
that actually need to be changed - this works well in many cases because 
the ODF schema is quite well structured with many named patterns, but 
unfortunately there are a few places where that isn't the case and large 
chunks needed to be copied to override them.

in the easy case, to add an attribute you just want to search for the 
corresponding element, which will have a "foo-attlist" named pattern, 
and then add another attribute like this:

  <rng:define name="draw-enhanced-geometry-attlist" combine="interleave">
    <rng:optional>
      <rng:attribute name="drawooo:sub-view-size">
        <rng:ref name="string"/>
      </rng:attribute>
    </rng:optional>
  </rng:define>

currently only the features that are actually exported in the unit tests 
have been added to the schema; there is still some work to do here to 
add everything; the crashtesting script also does ODF validation of all 
files and now also uses the custom schema, so those results will be 
interesting...

during this work we found a couple bugs where the schema and the export 
filter didn't agree; this led to updating at least one not-yet-accepted 
proposal at OASIS, and 2 bug fixes in LO code.

unfortunately it turned out that there are a lot of extensions already 
for which no proposal exists, and in many cases not even an entry on the 
Wiki [2], so clearly something like this extension schema is needed :)

[1] git grep TODO schema/libreoffice
[2] 
https://wiki.documentfoundation.org/Development/ODF_Implementer_Notes/List_of_LibreOffice_ODF_Extensions