Fix issues related to tree changes.

- Changing content[select] to more restrictive needs to clean
  up non matching nodes (we clear the child nodes now)
- Changes to fallback content needs to invalate
- Adding a content or shadow needs to invalidate

wip
Merge pull request #233 from arv/adding-content-or-shadow-needs-to-invalidate-renderer

Fix issues where we did not update the renderer after DOM changes