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.
Reviewed By: epriestley
Differential Revision: https://secure.phabricator.com/D15288