"View Source" button removed when exiting Inspector from Examples

[cvan]

1. Load https://aframe.io/examples/showcase/spheres-and-fog/
2. Click "Visual Inspector" button
3. Click "Back to Scene" button
4. Notice "Visual Inspector" and "View Source" buttons are missing, but there's an "Inspect Scene" button in the top-left corner

[ngokevin]

To solve this, we'd need to add a postMessage to detect Inspector exit.