Add observers for props

[janhohenheim]

Something like

props.observe("alertness", on_set_alertness);

fn on_set_alertness(change: On<MutateProp>) {
  ...
}

#[derive(EntityEvent)]
struct MutateProp {
  /// Entity holding the [`Props`]
  #[event_target]
  entity: Entity,
  key: Ustr,
  old_value: Value,
  new_value: Value,
}

whether props.get("alertness") should trigger this when such a call initializes the value or reinitializes it to another data type I do not know. Maybe?