Make QWaylandWindow::setVisible(false) work

Change-Id: I8f742f03819c7a626b23a92e211bd990a8d39969
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>