Add support for input and output of 'split position' parameter (number of entries
in second plot) for of-pie charts. In OOXML this uses the supported split-pos
tag. For ODF I added an extension in loext namespace for this parameter.
This commit also includes simple tests for the I/O functionality in OOXML and
ODF.
Change-Id: I00ff59db721867fa836eb99b6677350040d005dd
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/170666
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
