editor: add label removal possibilities

[jum-s]

While this feature is implemented by doing some duck typing a bit (aka depending on whether value is defined as updateOrRemoveLabel, it will update or remove label accordingly), but the cases are a bit complex if you want to add the delete button (it would simplify a lot not to have it, but it does follow claim editor mechanism).

[jum-s]

As undo feature adds some complexity, I added e4a2bfa2c to fix some difficult states one may end up in ie. add a new label, then delete another one, then select an other language should return a wrong labels list, such as: