Add some simple side bar functionality with a mock editor environment (#6104)
* Add some simple side bar functionality with a mock editor environment
* Add additional comments
* Minor review comments
- Add comments
- Use more constants
- StatelessWidget where state not required
- Fix typos
- Add `selectDevice` capability
- ...
* Move mock environment to test/stager scene
* Fix "Dark Theme" toggle in VS Code panel in stager scene
* Use defaultSpacing + remove custom split widgets
* Remove unused key parameter
* Update imports