treelistmodel: Track the item in the row

treelistmodel: Track the item in the row

That way, we can return the item even after the row is removed. This is
particularly relevant in ListItemFactory::unbind callbacks because they
often use gtk_tree_list_row_get_item() and user code never tracks
changes to this property.

A side effect of this is that the item will survive until the row gets
destroyed, but that's what users expect anyway, so we can live with it.

Related: #5646