Adds logic for locating and splitting text nodes.
Adds specific classes to offload tag/content specific logic.

Implements block promoting to body (including position choosing based
upon likely tag position within parent) and block splitting where we're
only a single depth down from the body child.

Expanded tests with many more cases, and added fixes for failed
scenarios.
Updated logic to specifically handling parent <p> tags, and now assume
compatibility with parent block types elswhere to allow use in a
variety of scenarios (td, details, blockquote etc...).

- Added mulit-level depth parsing.
- Updating usage of HTML doc in page content to be efficient.
- Removed now redundant PageContentTest cases.
- Made some include system fixes based upon testing.

Managed to do this in an API-compatible way although resuling output may
differ due to new dom handling in general, although user content is used
inline to remain as comptable as possible.