Cppcheck plugin: use QPlainTextEdit for non-richtext text display

Summary:
Uses less resources.

Also use QFontDatabase::systemFont(QFontDatabase::FixedFont)) over just
setting a font family "Monospace". should ensure a consistent monospace font
is used as in other places of the UI.

Test Plan: UI still shows the command as before.

Reviewers: KDevelop, antonanikin, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D14424