Call formatWindowTitle on the window title

According to QWidget::setWindowTitle documentation the QPA plugin is
responsible for adding the application name to the window title.
As it's supposed to be done for Unix platform the Wayland QPA also needs
to perform this task.

Task-number: QTBUG-56475
Change-Id: Ib261c68d08ca06d1ec4734c8c215a4ceb059fae3
Reviewed-by: Johan Helsing <johan.helsing@qt.io>
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@hawaiios.org>