Fixing races between parts of Tabster that want to focus something asynchronously.

[mshoho]

Tabster does some focus movements asynchronously. Restorer restores lost focus asynchronously (to give the DOM the opportunity to finish the changes), Groupper handles Esc asynchronously (to give the application ability to handle Esc inside the Groupper).

This PR fixes the conflicts when both Groupper and Restorer want to move focus at the same time.