Display figure brackets in plain text

Summary: RTF files can contain text with figure brackets, i.e. C++ code. This fix mentions the case when figure brackets should be handle as plain text and not as a token of the beginning or ending of the group of control words.

Reviewers: pvuorela, denexter, dcaliste, davidllewellynjones

Reviewed By: pvuorela

Subscribers: #calligra:_3.0, denexter, davidllewellynjones, Calligra-Devel-list, pvuorela

Tags: #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D25541