DOCX export tries to write accurately both <w:br> and
<w:pageBreakBefore>, while DOC/RTF prefers just the page break before
paragraph attribute. Given that these later two exporters are not
adapted to write their equivalent of <w:br> at all, add a virtual method
to determine if writing page breaks as "page break before" is preferred
or not, that'll give the expected RTF export result.

This conditionally reverts commit
a31fbb53 (fdo#74566:DOCX: Preservation
<w:br> tag for Break to Next Page, 2014-02-06).

Change-Id: I4a5dd295b07739a570cd8cc6709561e64d031273