fdo#83464 RTF import: \plain should reset \rtlch and \hich as well

Regression from 36246aa9fb57c9fe4e546c91a8274d8828b1424e (RTF import:
fix handling of \loch \hich \dbch \ltrch \rtlch, 2014-06-03), now that
\f no longer means the ascii font name, \plain should reset a few more
parser state members.

(cherry picked from commit 69bf37136c1386c7220f38a896aa9865bdae665f)

Conflicts:
	sw/qa/extras/rtfimport/rtfimport.cxx
	writerfilter/source/rtftok/rtfdocumentimpl.cxx

Change-Id: Ia6b0bdf25563dd5300e108b46731751f0886f67d
Reviewed-on: https://gerrit.libreoffice.org/12034
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>