This applies the proper CSS child ordering semantics using GtkCssNode to
GtkListBox. You can now use :first-child, :last-child, :nth-child(), and
:last-nth-child() selectors.

For example, this allows styling row separators using CSS while ignoring
the separator on the last row.

 GtkListBoxRow {
   border-bottom: 1px solid @borders;
 }
 GtkListBoxRow:last-child {
   border-bottom: none;
 }

When the sort ordering of the listbox changes, we also update the CSS
node ordering.