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