Fix leaf block removal in the backend (#12005)

* Fix leaf block removal in the backend

The fix introduced the new 'removal' method for the backend leaves set
and the improvement of the undo features.

* Update docs

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <[email protected]>

* Fix docs typo

* On block block removal the new children list should be persisted.

* Align leaves set removal tests to the new interface

Co-authored-by: Bastian Köcher <[email protected]>