Add special support for block characters
Summary:
Similar to the line chars, block chars look much better
if we handle them ourselves.
Test Plan: cat tests/boxes.txt and cat tests/UTF-8-demo.txt
Reviewers: Konsole, hindenburg
Reviewed By: Konsole, hindenburg
Subscribers: konsole-devel, Konsole
Tags: Konsole
Differential Revision: https://phabricator.kde.org/D17294