dbus: Automatically register objects
When iterating collections and items, immediately register them if
needed. That way if a collection is added outside of the D-Bus service,
it will automatically be registered the next time a client requests the
Collections property.
Without this, the login keyring created by the PAM module is never
registered since it happens after the collection initialization. When
clients query Collections, they'll see the new keyring's object path.
However, attempting to use it will fail since the collection object
hasn't been registered.
Fixes: #137