Add support for `cemetery=sector`

**Use case** When one is in a cemetery, he may want to tag the sectors he has seen. For that he needs to (1) be able to add a new sector and (2) to be able to see existing data. Both are not currently possible. **Proposed Solution** (1) Allow adding `cemetery=sector` via the things overlay with fields for `ref` and `name`. (2) Render `ref` and/or `name` for already tagged nodes and areas.