Improve PhabricatorPropertyListView and add section headers

Summary:

• For Drydock, I want to add section headers to separate user-defined attributes from global attributes.
• Some day for Differential, I want to add "Summary" and "Test Plan" section headers.
• Clean up some stuff a bit; drop the multiple APIs for setting text content. Explicitly disallow appendChild().
• Build out the UIExample a bit.

Test Plan:
{F24821}

{F24822}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T2015

Differential Revision: https://secure.phabricator.com/D4000