Add setBackground to PHUIIconView
Summary: Ref T11132. Adds a background color option to PHUIIconView, for use whereever, and NUX. Also normalize icon placement for mixed image/icon result list.
Test Plan: Test in UIExamples, and Global Settings.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T11132
Differential Revision: https://secure.phabricator.com/D16424