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