tree-column-view: don't steal children

When mousing over a dialog that uses this kind of view (e.g. one created with the test command in #54 (comment 1726876) ), I see a lot of these errors on the console: gtk_list_item_set_child: assertion 'child == NULL || gtk_widget_get_parent (child) == NULL' failed I think we can hit this callback more than once, and doing so means we try to set the list item's child again; Gtk doesn't like that, because we're effectively trying to set the parent of an item that already has one (it doesn't check if we're just trying to set the same parent, it just complains any time you try to set something as the child that already has a parent).

Signed-off-by: Adam Williamson awilliam@redhat.com