R1:10a9f2a15f27

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…