DOCX import: fix FooterBodyDistance for first pages

The root cause was that code in
SectionPropertyMap::PrepareHeaderFooterProperties() saved member
variables on the stack, modified them, then restored them at the end of
the method, but forgot to restore m_nBottomMargin there.

The result of this was that First Page's FooterBodyDistance got set to
zero, which is not the same as Default Style's FooterBodyDistance (150),
so on export sw::util::IsPlausableSingleWordSection() returned false,
which turned the page break into a section break.

Change-Id: I1afa2603de1313ae1e2aaefd4b7c771fce27e21e
Clean up SectionPropertyMap::PrepareHeaderFooterProperties()

If you copy the class members on the stack and tweak those (and not
class members), then there is no need to manually restore the copy at
the end of the method.

Change-Id: I42b50cd6b99b6b277dd9809b40385ef6512ef141