RTF import: fix handling of \loch \hich \dbch \ltrch \rtlch

The logic is not immediately obvious from the RTF spec; let's do what
the editengine RTF import does, but without the unnecessary complexity.

(cherry picked from commit 36246aa9fb57c9fe4e546c91a8274d8828b1424e)

Conflicts:
	writerfilter/source/rtftok/rtfdocumentimpl.cxx
	writerfilter/source/rtftok/rtfdocumentimpl.hxx

Change-Id: I60e69130e6e5aed1f5d237f64b1656c3141e402a
Reviewed-on: https://gerrit.libreoffice.org/9634
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>