Port improvements from D7 Edit to D8

Merci beaucoup!

1. Will do, but not here because it's non-trivial. Follow-up issue: #1943776: In-place editors (Create.js PropertyEditor widgets) should be loaded lazily.
2. Over at #1943696-4: [meta] Bring D7 & D8 Edit in sync; modify broken/revert unnecessary patches, I backported D8 changes to D7. Those changes made this hack obsolete.
3. This was solved incorrectly in D7. It's already been solved the correct way in D8. The correct way involves not adding more hacks, but instead cleaning up Create.js PropertyEditor widgets to do what they're supposed to do: be dumb and not determine themselves when they're activated. That was the problem here, really: Create.js PropertyEditor widgets decided themselves when they were going to be activated, which makes zero sense.
4. Problem no longer exists in D8.
5. No longer relevant, as this is no longer owned by the Edit module.
6. Done.
7. Irrelevant for D8; D8 doesn't have such a block.

Problems/clean-ups I encountered while working on D7 Edit, I've also forward ported to D8:

1. #1891246-6: White background through JS
2. #1894298: When editing body field with ckeditor teaser == body text

This patch fixes a bunch of smallish bugs:

• ensures body field modifications don't also modify summaries
• ensures in-place editing also works well for light-on-dark themes
• ensures Drupal behaviors are detached for fields edited via in-place forms
• ensures Drupal 8 will be able to handle display:inline fields like Drupal 7's Edit can (Drupal 8 does not yet have any such fields, because the e.g. the node title is not yet a proper "entity field", it's an "extra field"; this ensures that it will work well)

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