DOC/RTF export: fix handling of column breaks when there is only one column

The first real part of commit 4d5c193b2fd38c6cab049fcb97189462fff0fddb
(INTEGRATION: CWS limerickfilterteam08 (1.64.6); FILE MERGED,
2003-09-01) tweaked the DOC export, so that in case there is only one
column, the column break is not exported: this way the Writer and Word
layout matches, because Word handles that situation by handling the
break as a page one, but Writer layout ignores it.

On import, the DOC filter changes a column break to a page break in that
situation, so visually the roundtrip is OK. The RTF filter does the
same: the tokenizer turns a column break into a page one if necessary,
and on export then we can ignore such a column break.

However, the DOCX filter is different: there we don't tweak the column
break on import, so we want to keep it on export as well. (A perfect
solution for this would be one more layout compat option, then filters
can stop tweaking the break types.)

Recently commit 78c5cbc720993e9cc01fceadd73678ed1a45d37f (fdo#74153 :
Preservation of Column Break with column_count = 0, 2014-02-03) turned
on the export of such column breaks in the exporter unconditionally, and
this broke the above logic of DOC/RTF roundtrip.  Fix this by adding a
virtual method where each format can decide what it wants.

Change-Id: I807c2fdc02aefc20ffbb4a4dbbf3845ecad81bca