[META] Accordion effect for paragraphs

Now that the main issue is committed, we are missing lots of follow-ups to care in this issue.
Identified:
- Provide demo content
- JS test
- Indicating what is a title and what is a body in the UI.
- Validate that the childrens are mod 2.
- Checkbox for auto-collapse, so only one item can be open. If not checked, multiple can be opened.
- Figure out how to deal with multiple levels of nesting.
- Generalize container child field identification
- Reconsider what plugins (with child relationship) are on generic container and if they should have an effect without enabling them per instance. We should identify what plugins OWN layout, thus only one of these can be enabled per instance.

Please create the follow-ups and link them. :-)