We need a backend class representing the contents of the watchlist_label table:
- Add label
- Delete set of labels by ID
- Rename label
- List defined labels for user
Noting that of these four operations, only "delete label" has a referential integrity dependency on the watchlist_label_member table. WatchedItemStore could potentially manage watchlist_label_member, but asking it to also manage watchlist_label seems a step too far. It's already 1800 lines of code.