Limit IconView to only display top level of model.
This commit facilitates using a multi-branch treestore with an icon view, as items on the models branches are now ignored.
This will make it easier to complete thunar!674 by removing the need to deal with these branches in Thunar.
Based on upstream code from Michael Natterer https://gitlab.gnome.org/mitch
Gtk Commit e4654327 (https://gitlab.gnome.org/GNOME/gtk/-/commit/e4654327df31bc1bd0fd3be4433ee008085ebe95?view=parallel)