Add new api Order functionality to update line items
I'm pretty sure that despite @JoeMurray's optimism the current apiv3 Order.create doesn't support this. @monish.deb & I discussed adding it back here
https://github.com/civicrm/civicrm-core/pull/11380
I don't think we should add it o apiv3 now - I think we should write a v4 api that does it. I think the object oriented approach will allow us to come up with cleaner methods. I'm kinda keen to try to build those up on a ORDER pseudoBAO class & play a bit before we expose in the api so we can learn about about what works & doesn't before locking it in