Spawn and despawn billboards

[nixpulvis]

Using Visibility::Hidden isn't enough to deal with the fact that they are children of each system. What was I thinking!

Using similar logic related to #43, we should be able to despawn and respawn billboards even while their parent system remains around.

[nixpulvis]

fn update(commands, camera: Query<&Camera>, systems: Query<(Entity, &System)>) {
    // forall systems, get camera distance then if:
    // distance > D remove label
    // else insert or reposition the label
}

[nixpulvis]

Fixed in ddbfd91