Use QStringList::join(QChar) overload where applicable [QtWidgets]

Use QStringList::join(QChar) overload where applicable [QtWidgets]

This is an automated change performing the following replacements:
                    join\("(.)"\) -> join('\1')
  join\(QLatin1String\("(.)"\)\)  -> join(QLatin1Char('\1'))
  join\(QStringLiteral\("(.)"\)\) -> join(QLatin1Char('\1'))

Change-Id: Ia30048e0c40967dc86a4e4ad26ac02ab67519096
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>