Implement some cleanup functions

As now all records are accessed by relations (anime-/mangalist or record
relations like prequels, adaptations etc.) all records in the database
that are not related somehow are "dead" records. So I implemented some
functions to delete them:
- deleteFrom(Anime/Manga)List() now checks if the deleted record is
  still related to other records and deletes it if not
- a full sync now also checks for unrelated records and deletes them