RTF import: handle RTF_FTNSEP and RTF_CHFTNSEP

Commit 330b860205c7ba69dd6603f65324d0f89ad9cd5f (fdo#68787 DOCX import:
handle when w:separator is missing for footnotes, 2013-09-04) disabled
footnote separator by default in dmapper, as the OOXML tokenizer always
provided a uFtnEdnSep in case a separator was wanted.

Let the RTF tokenizer do the same, this way we're in sync with Word
again: if RTF_CHFTNSEP is in RTF_FTNSEP, then we show the separator,
otherwise we don't.

Change-Id: I74b46c5d71227682e093695336dc9eb6fde22121