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

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

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

Change-Id: Id8d43c1b1ca9cd3aa0b4eedca6709b6287a9bc98
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>