[web] Additional state required for keyboard support for typeahead.

Summary: We need extra state for controlling currently chosen button in typeahead overlay and for user actions (Escape for closing it, or Enter for accepting).

Test Plan:
App runs.
Extra states used in future diffs, where they will be tested.

Reviewers: tomek, kamil, atul

Reviewed By: tomek

Subscribers: ashoat, atul, kamil, tomek

Differential Revision: https://phab.comm.dev/D5818