fdo#84645 RTF import: set DontBalanceTextColumns=true for the last section ...

... if it has multiple columns. See commit
d185204737031955c56a24356ed003d342548434 (DOCX import: set
DontBalanceTextColumns=true for the last section, 2014-07-17) for the
DOCX equivalent of this problem; this just adapts the RTF tokenizer to
dmapper.

(cherry picked from commit d185204737031955c56a24356ed003d342548434)
(cherry picked from commit f4bb73164a51ec83fe1d5975b1232d35f8a9e88a)

Change-Id: Ib30f9b386e204b8b2987832ab17ee0cc53b3f0bc
Reviewed-on: https://gerrit.libreoffice.org/12462
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>