This leaves still a small mismatch in the Editor API which ideally would be removed here by recoding the signal values: `setMetadata` takes for `removedProperties` a list of string with the names of the metadata, `{?Array.<!string>}`. But the signal carries the removed metadata entries as `removedProperties:?{attributes:string}`, which I assume users of the API might find strange and would rather expect that as `removedProperties:?Array.<!string>` as well.
Perhaps the signal `signalMetadataUpdated` should be emitted from the very begin with `removedProperties:?Array.<!string>`, because each and any consumer of that signal that is interested in the removed metadata otherwise would have to parse that `attributes` property manually.