Support usage within Block Field widgets

Problem/Motivation

It would be great if this module could be used together with the Block Field module. This would allow, for example, an entity field (let's call it "Sidebar content", for instance) to point to very different things in nature, such as some nodes, media items, or even views. All that could be editor-selectable, and draggable within the block field widget.

Currently the module doesn't work in that scenario though, since the block configuration form has a different structure as when used in a normal Drupal block placement, which breaks ::processTable() and similar.

Proposed resolution

Tweak the code so it is compatible to both normal Drupal block placement config forms, and block field widget forms.

Remaining tasks

User interface changes

API changes

Data model changes