The core of this change is that RtfAttributeOutput::PostitField() now
uses WriteOutliner() to handle comment text, instead of trying to output
the plain text as-is.

To have working roundtrip of the comment text, a few more changes are
needed:

1) When emptying the Styles buffer, also empty the StylesEnd one.
2) Annotations don't want a trailing \par, so don't write one for the
last paragraph.
3) Inform the MSWord_SdrAttrIter about where the editeng content will
end up, and make WriteOutliner() accessible outside RtfSdrExport.

Change-Id: I9cbcf4ce5dc3a099d310c6f321ea8e52f8644f9b
(cherry picked from commit 7060525a)