Save transaction status in order meta (526)

[stracker-phil]

Description

This PR adds a new order-meta value with the PayPal transaction status. The purpose is helping third-party code to understand if an order is pending payment, authorized, or captured or refunded/voided.

When the transaction status changes, a new WP action fires to allow third-party code to take immediate action:

/**
 * Fires after the PayPal order status value was updated in the order meta table.
 *
 * @param WC_Abstract_Order $wc_order   The WooCommerce order. 
 * @param Order             $pp_order   The PayPal order.
 * @param string            $new_status The new status.
 * @param string            $old_status The old status.
 */
do_action(
    'woocommerce_paypal_payments_order_status_changed',
    $wc_order,
    $order,
    $new_status,
    $old_status
);

Implementation

This PR introduces a new OrderMetaManager class that contains the order-meta logic (reading and writing):

$meta = new OrderMetaManager( $wc_order, $order );
$meta->update_status();  // Saves the $order->status value to the WC order-meta table
$meta->persist();

$status = $meta->get_status();