Add entity destruction detection to observers

[RiscadoA]

Problem

Currently observers can detect removal of components, but not destruction of the entities themselves.

Solution

As we have .onRemove<Foo>, we should also have .onDestroy for querying over destroyed entities.
I'm not sure on whether these observers should run before or after the .onRemove observers for the components on the destroyed entities.

This could be also be used to implement #1418.