Add an Order State field / filter to Views

Currently you can display and filter by the order status, but you cannot easily display and filter by the state of an order. There isn't technically an order state property for the order, but instead the state is derived from the current order status. To load the order state from an order status, you use a simple code:

  $order_status = commerce_order_status_load($order->status);
  return $order_status->state;
  // Or load the state object to extract its title.
  $order_state = commerce_order_state_load($order_status->state);

The filter should be able to extend the order status filter and just automatically select the order statuses for the chosen state.