Add paragraph_view from simplelayout.types.common

simplelayout.types.common is now optional.
Packages such as ftw.contentpage and ftw.book expect that there is a `paragraph_view` which usually is used for blocks, redirecting to the parent simplelayout container.
Since we've dropped s.t.common, this view is missing.

This PR adds the view to simplelayout.base, so that all those packages work again without changes needed.

Includes:
- Splitting up testing layer, so that we can test `paragraph_view` without loading s.t.c ZCML (which also still has one)
  - a little bit of cleanup (removed unused imports)
- Adding `paragraph_view`

// @maethu