password: Add secret_password_search* functions

Previously there were no functions in the simple API that return matched attributes other than the secret value, though there were needs to augument user input with additional information (such as completing web forms).

This adds a set of functions which wrap secret_service_search*. Note that the type of the return value is GHashTable, not SecretItem, because SecretItem is a subclass of GDBusProxy, which we want to hide from the simple API.

Fixes #16 (closed)