Handle composite object notifications better

[castleguarders]

Example: When a new person is detected in a scene, along with two cars that have been parked for a while. Two sets of notifications are issued, once for the person, another for the parked cars. The cars would normally have not resulted in a notification as they would be recognized as already having been there for over the configured threshold. However because the 'person' is new, the cars are also causing a redundant alert.