This was handling XML elements at 3 different levels; add some more
context classes to handle the nesting better.

There was one tricky case: the ORptExport::exportContainer()
can export a office:report element inside office:body
inside report:sub-document, but i don't know how to
create a report like that.

This was previously handled on import via the fallback to
SvXMLImportContext (created for office:body) calling
ORptFilter::CreateContext(), but now it needs explicit handling
in OXMLSubDocument::CreateChildContext_().

Change-Id: I839986599b2b936f57bb65da355ac1be811fe999