Fixes#5231
- Add stdin handling. Has to be at UI level so add plumb through handling
- Add unit tests
- Add new StdIn component.
Testing:
Unit Tests and manual testing of following:
- Prompt for password using `getpass` in python.
- Password prompt is hidden since "password" is true.
- Hit enter, it completes
- prompt, stop cell running, StdIn disappears
- prompt, hit escape, stdIn disappears and stdIn request is handled.
Issues: focus isn't always set to the input even though we call focus.
Will investigate this further.
