Docxcompose

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- Add support for Smart Art (fixes 23)
- Correctly handle mapped styles in restart_first_numbering. [njohner]

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- Make Doc Properties case-insensitive. [buchi]

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- Add support for complex fields with fieldname split into several runs. [njohner]
- Add support for date format switches. [njohner]

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- Support updating complex properties with no existing value. [deiferni]

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- Add method to nullify a docproperty. [deiferni]

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- Handle embedded images that also have an external reference.
[buchi]
- Fix renumbering of non-visual image and drawing properties.
[buchi]