Hiding tracked objects when not needed

When the app first starts tracking, we show the trackable planes and point clouds. This is useful feedback to the user when the app first starts and subsequently when placing an object. But once we have objects placed in the scene, these trackable visualizations can be distracting and unwanted. Let's only show the object while in PlaceObject-mode and hide them after at least one virtual object has been placed.

In AR Foundation, hiding the trackables requires two separate things: hiding the existing trackables that have already been detected, and preventing new trackables from being detected and visualized. We will implement both.

To implement this, we can write a separate component on PlaceObject mode that shows the trackables when enabled and hides them when disabled. Follow these steps:

1. Create a new C# script in your Scripts/ folder named ShowTrackablesOnEnable and open it for editing.
2. At the top of the class, add variable...