Events for stock changes

[steroberts89]

I keep coming back to this idea, I think it would be great to get an event added for when the stock level of a product is set/changed.

You could accomplish this with finalising events (On order complete, on refund), but I don't think there is one for the "On Content" stock level editor, and this just feels dirty.

Reasons I'd want this, we have a faceted product view, and we only want to show products which are in stock, which means asking the database for each product on the display. If we had an event to hook into for when stock levels change, I would just store the stock level in a lucene index.