Docx reader: clean up parStyle processing.

This gets rid of `divAttrToContainers`: an internal convenience function
which had become pretty inconvenient. Rather than converting classes and
indentations to string lists and back, we deal with the `pPr` attribute
directly.
Merge pull request #1388 from jkr/divattrToCont-cleanup

Docx reader: clean up parStyle processing.