This literal was deprecated in 5.14.0. Not to mention it wasn't
documented as part of the API either: see the 5.14.0 changelog here:
https://code.qt.io/cgit/qt/qtbase.git/tree/dist/changes-5.14.0?h=v5.14.0
On Qt 6.0 this define is removed entirely. To stay forward compatible,
we can make use of QStringLiteral instead.
Lambda expressions with uncaptured constants were leading to errors,
and there were also some warnings about deprecated functions
(QFontMetrics::width and inet_ntoa).