This is causing FilePicker to log a bunch of debug noise due to the
missing support for tree models in SortingProxyModel and it's not
helping anyone so let's just disable it.
This needs fixing in SortingProxyModel.
A MultiView is a combination of ItemView, TableView and ColumnsView
smashed into a single widget. You can switch between the view modes
by calling MultiView::set_view_mode().
Note that MultiView inherits from StackWidget, not AbstractView.
That's purely for practical reasons, although I'm not entirely sure
if there would be some benefit to having it inherit from AbstractView.
