Expand scope of addActionItem in PHUIHeaderView

Summary: Gives a bit more flexibility to add anything to the right side of PHUIHeaderView.

Test Plan: Test Maniphest, Workboards, Project Home, Differential. Grep for addActionIcon use. Fixes T10518

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10518

Differential Revision: https://secure.phabricator.com/D15402