* Turn secret_service_search() and friends into a more convenient
API, so that callers can get attributes, unlocking, and secrets
if so desired.
* Also support retrieving either one secret, or all of them.
* The SecretService methods aren't used as frequently as the
password methods, and it's not really necessary to have a whole
bunch of extra varargs functions.
* Add varargs functions for building a GHashTable of attributes
* This allows callers to use a custom SecretService derived
type with the rest of libsecret
* The default way to get a SecretService is via secret_service_get()
which remains simple without the GType parameter
* Schemas can also be created dynamically
* Correct introspection types for hash tables passed as args
* Validate argument hash tables
* Add some predefined schemas
* Allow allocation of pageable passwords from lookup functions
