Add Inventory events for developers to hook into

[engram-design]

This is in relation to the Back in Stock plugin, but I'm sure there's a general need for this. There seems to be no events raised when handling inventory items.

Previously, as stock was managed at the Variant level, listening to Variant::EVENT_BEFORE_SAVE was a straightforward way to know about stock changes. However, as stock is now managed as dedicated inventory, we should be listening to the stock level changes on that, for our plugin to be able to notify users that a variant's stock has been added and is available for sale.

I would propose the usual update/delete events, similar to other models.