Improve focus managmenet

[eschiebel]

This makes three changes to help improve focus management

1. if the editor associated with an a11y checker is removed, close the checker
2. when the checker is closed, focus the associated editor
3. add the data-mce-checker attribute to the checker's Tray component. Consuming applications may use this to detect that even though focus may have left the editor, it is still within a compnent that can be considered a paart of the editor's ecosystem.