EDIT

copied from #7912:

Improve display of top level items in checkbox tree viewer
OK, this works. Only it feels a bit strange that there is a checkbox which is always empty and can not be clicked and at the same time all the childrens are selected, e.g.:

So best would be to handle this checkbox as standard checkbox with in trees with unchecked=no child selected, checked=all children selected, tristate=some children selected. Alternatively, if possible, we could simply remove the checkbox.

Set gray state

Even if greying the checkbox does not work on Ubuntu for some reason I think we should still try to do it and see if it works e.g. on Windows. A grey checkbox is more intuitive then italics. But of course we will keep the italics too to make sure that the dialog generally works on all systems.