When a document ends with \sect it's possible that a spurious page break
is created.  In fact the spurious page break is always created by the
RTF importer, sometimes it is deleted again by
DomainMapper_Impl::RemoveLastParagraph() and sometimes not.

It is created because on the final \sect RTFDocumentImpl::sectBreak()
still calls startSectionGroup(), and the popState() for the \rtf1 group
then calls sectBreak() another time.

To prevent this, do not call startSectionGroup() from sectBreak() but
instead from setNeedSect(), and ensure that it is called as soon as
anything after \sect is read.

One unit test fails because the \page is not handled properly: the
conversion to \skbpage \sect \skbnone is not correct, because the \skb*
keywords are an exception and affect the \sect that precedes them, not
the following one; sending the \skbpage later unfortunately requires
additional cleanup later.

Change-Id: I3c1a3bceb2c8b75bbecdc748170562451ce5f5c3