fdo#73596 [DOCX] Multiple Columns in Index

The number of columns in an Index is given by the "\c" switch.
During export, added section breaks before and after the Index;
this is to preserve the layout of the round tripped file in MS Office.

Modified MSWordExportBase::NoPageBreakSection so that
an incorrect "nextPage" section break is not exported.

Conflicts:
	sw/qa/extras/ooxmlexport/ooxmlexport.cxx
Reviewed on:
	https://gerrit.libreoffice.org/8047

Change-Id: I364e19d6739c4c6a60e3c605044fc1fafd9147d3