Allow adding custom 'action buttons' in order item metabox

Similar to #6305 only we are adding 'action buttons' instead of the 'add line buttons'. These buttons would site before the blue 'calculate' buttons, but after the grey 'add line item(s)', 'add tax' and 'add refund' buttons. In my use case I am adding a 'add coupon' button, so that an admin can apply a coupon to an order, for proper reporting. I am sure there are other use cases though. I figure here is the best location, since we can add blue or grey buttons at this location. Also, I corrected the indention problem in on the blue buttons, so that they match indention of the greys.