Moved Viewer functionality to CesiumWidget class

[jfayot]

Description

This is a proposal to move Viewer functionalities not related to widgets to CesiumWidget class as proposed in #11967.

Issue number and link

Fixes #11967

Testing plan

• Updated ViewerSpec.js
• Updated CesiumWidgetSpec.js
• Updated Cesium Widget sandcastle

I kept duplicate tests in ViewerSpec, may be some can be removed as they are implemented in CesiumWidgetSpec now?

Author checklist

• [x] I have submitted a Contributor License Agreement
• [x] I have added my name to CONTRIBUTORS.md
• [x] I have updated CHANGES.md with a short summary of my change
• [x] I have added or updated unit tests to ensure consistent code coverage
• [x] I have updated the inline documentation, and included code examples where relevant
• [x] I have performed a self-review of my code