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