Add ability to setActionList to a PHUIHeaderView

Summary: We're using this a little more, so I'd prefer less copy-pasta and one place to manage the UI. Maybe add a caret?

Test Plan: grep for 'Actions', test Phriction, Diviner, ect, Action Menus.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15288