R1:998d43e828b0

Add a "toggle-class" behavior

Summary: The new menu stuff needs this but it was easy to pull out on its own.

Test Plan: Cliked UI example buttons.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1569

Differential Revision: https://secure.phabricator.com/D3104