Both items and collections accessed through secret service API might be
locked. Secrets of locked items cannot be accessed. Locked collection
cannot be modified.

Implement unlocking as recommended by secret service API specification:

* The client application should act as if it must unlock each item
  individually.
* A client application should always be ready to unlock the items for
  the secrets it needs, or objects it must modify.
* It must not assume that an item is already unlocked for whatever
  reason.

Fixes issue #175.