To trigger this, cal KDirModel::openUrl(rootUrl, KDirModel::ShowRoot)
KDirModel has always been lacking a openUrl method anyway, it's always
been weird to create a KDirModel but then call
dirModel->dirLister()->openUrl().
This feature has been requested by Raphael Rosch who is reimplementing
the konqueror sidebar tree on top of KDirModel. Having a visible item
for "/" is necessary in order to be able to click on it and have it
listed in the main file view.