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.

Change-Id: Ia6b0bdf25563dd5300e108b46731751f0886f67d