ODT export uses nested spans to represent a combination of named character
style + automatic style (instead of having a parent for the automatic style),
but librevenge doesn't allow nested spans, so handle this explicitly.

The alternative would have been to remember the attribute list as member
data, but the underlying SvXMLAttributeList is reused after
startElement() returns, so it by the time characters() is invoked, it
won't have the data we need anymore. (Would be a trade-off between doing
the attributes -> property list conversion exactly once or depending the
number of characters() invocation anyway.)

Change-Id: I1dd2f060c421c126340db471a257093b30431d17
Reviewed-on: https://gerrit.libreoffice.org/42046Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>