Modal.js should consider shadowRoot

[thomson02]

consider the possibility that document.activeElement could be a shadowRoot. In this case recursively look inside shadowRoots until the actual focused child is found.

• [ x] Bug fix
• New feature
• Chore
• Breaking change
• [ x] There is an open issue which this change addresses
• [x ] I have read the CONTRIBUTING document.
• [ x] My commits follow the Git Commit Guidelines
• [ x] My code follows the code style of this project.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• My change requires a change to Typescript typings.
  • I have updated the typings accordingly.
• I have added tests to cover my changes.
• [x ] All new and existing tests passed.

Fixes #2777