SecretsManager: dependency management

[rpbouman]

Catalogs can be explicitly linked to secrets.
It would make sense to:

• add something so you can see from inside the secrets dialog where it is used. (currently, only catalogs)
• a delete of a secret should prompt the user if it is user somewhere. Possible options:
  • cancel (don't delete the secret after all)
  • delete all fields that references the secret
  • delete all objects that reference the secret.
  • just delete the secret, allowing the references to be broken.