Apply drag & drop mode to all top level fields

Problem/Motivation

The drag & drop mode currently only switches one top level field.

So if i have two top level fields - say a content and a sidebar (each with paragraphs) - i can't drag & drop a paragraph from the sidebar into the content area. even if it is a supported paragraph type.

Proposed resolution

Let's switch the host entity as a whole into drag & drop mode and allow dragging across anything. :-)

Remaining tasks

User interface changes

API changes

Data model changes