Move PhabricatorObjectItem to PHUIObjectItem, add 'plain' setting for lists.

Summary: Adds plain support for object lists that just look like lists

Test Plan: review UIexamples and a number of other applications

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6922