There was not any event fired for when an entity despawns for reasons
other than death, EG when a non-persistent entity is removed because no
player is nearby (RemoveWhenFarAway). This commit adds a simple
entity-despawn-event to fill this void, to be used with a matching CB
commit that will fire this event.

Or, at least, the majority of other events in the Entity category. It
seems the standard is to use the word 'Called' and end with a period
(full-stop/dot).

A way to identify *why* the entity despawned is very useful.

The entity despawned because it died

An extremely vague DespawnReason that could be caused by a variety of
things... but it's better than just DEFAULT.