Event for when a node is forgotten

It might be useful to have an event for when a node is forgotten. It should be fired after the tree is replaced rather than while the tree is being replaced.

Some options:

* Event subscription per node
* Single subscription on a project and then a single fire with an array with all the forgotten nodes (probably this)

I’d need to look at the code again, but I think this would be easy to do. There shouldn’t be any performance overhead of this feature unless people subscribe.