Split HtmlMainFragment off of HtmlFragment for the page body

Problem/Motivation

We want to move the page processing system to using objects, notably HtmlFragment and friends. However, there are some things that only the main body (aka controller) should be able to define. Most notably the response code. Possibly the title as well, although Blocks have titles.

Proposed resolution

Create a HtmlMainFragment class that implements HtmlFragmentInterface plus extra methods (with interface?) for additional main-area-only content.

Then when we convert a controller result render array to an object, convert it to HtmlMainFragment instead of HtmlFragment.

Remaining tasks

Figure out what all should be on the main fragment only.

User interface changes

None.

API changes

Minor.