Additional notification, allowing customization of display-only order totals

[lat9]

Some customizations might include additional order-totals that should be displayed, but considered read-only.

EO 4.4.0 and later will issue the EDIT_ORDERS_DISPLAY_ONLY_TOTALS notification:

    $display_only_totals_list = '';
    $zco_notifier->notify('EDIT_ORDERS_DISPLAY_ONLY_TOTALS', '', $display_only_totals_list);

... allowing watching observers to supply a comma-separated list, e.g. ot_balance_due, ot_payments_received, of additional order totals that should be displayed but not provided as inputs.