Daiki Ueno authored Jun 17, 2019 and Daiki Ueno committed Jul 18, 2019

This is a ground work for adding a local storage backend.  As
SecretItem is derived from GDBusProxy, it cannot be simply exposed to
the application through the secret_password_search() if the item is
not backed by the DBus API.  This adds an abstract interface
representing a read-only view of a secret item for that purpose.