Don't use deprecated toAscii() method

Summary:
QChar::toAscii() and QString::toAscii() are deprecated in favor of
QChar::toLatin1() and QString::toLatin1() respectively.

Reviewers: #kwin, apol

Reviewed By: apol

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22374