Properly handle despawned entities with Process component

[edouardpoitras]

We need to ensure that if an entity with a Process component is despawned, that the underlying process is killed (or at least document the behavior appropriately).

On Linux, it seems the process would continue running in the background and no ProcessCompletedEvent or ProcessOutputEvent would be generated any more. Once Bevy exits, the process would terminate.

This behavior could be different on Windows, remains to be tested.

[edouardpoitras]

Relevant resources on how to act on entities being despawned:

• Add way to hook into entity despawns bevyengine/bevy#2031
• Component Lifecycle Hooks and a Deferred World bevyengine/bevy#10756
• Generalised ECS reactivity with Observers bevyengine/bevy#10839

[TimJentzsch]

The component lifecycle hooks have been merged, so we can probably tackle this once Bevy 0.14 is released