It is retrieved with inheritance and if set takes precedence
over the variable `anki-editor-prepend-heading`.
Also add a function to toggle it and change the behavior
of `anki-editor-toggle-format` to match it: Now every call
to the toggling functions will actually toggle the value in
effect and an entry property is only set if necessary.
It improves upon `anki-editor-delete-notes` when deleting note at
point, allowing aribtrary position in the note when calling it,
and also allowing deletion in Org. `anki-editor-delete-notes` is
removed.