Add header icons to PHUIPropertyListView
Summary: Adds summary (description) and test plan icons to make these area's more unique and differentiated over general sections.
Test Plan: Test a diff, a commit, a task
Reviewers: epriestley, btrahan
Reviewed By: epriestley
CC: Korvin, epriestley, aran
Differential Revision: https://secure.phabricator.com/D7493