Trigger event on change of Order's Paid Status

[nfourtythree]

When an order's paid status changes trigger an event.

An implementation strategy is required for this due to the number of variables as to why the paid status might change.

As just one example an order might go from paid to unpaid for the following reasons:

• Order was fully or partially refunded
• A transaction was deleted
• Order was edited in the control panel
• Order was edited programmatically