change theme json format, use meta object enum names for editor colors

I wanted to create some export in KTextEditor and stumbled over the manually added keys here. I think it makes sense to do the same as for the styles and just use the enum meta info. I added a backwards compatible code path, that is taken if no new key is used. => I hadn't to change the themes and all still works

@dhaumann @ngonzalez

Would that be ok for everybody?