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