Internal paste rules and type conversions

Separate out paste rules into external only rules, and 'all'
rules which apply to both external and internal pastes.

Also add support for model type conversions, e.g. headings to paragraphs.

Change-Id: Id995f41d5f5bca4b8f31306e3b3ce25745e30ecc
Merge "Internal paste rules and type conversions"