PhabricatorActionHeaderView v0
Summary: Initial pass at an action header. The idea is to support current and future planned needs in 'headers' with various colors and icons. The overall goal here is to keep markup light and allow other classes to wrap and extend with more specific features.
Test Plan: Tested UIExamples and Workboards.
Reviewers: epriestley, btrahan
Reviewed By: epriestley
CC: aran, Korvin, AnhNhan
Differential Revision: https://secure.phabricator.com/D5585
