libnm: make sync/async APIs more GLib-like

libnm: make sync/async APIs more GLib-like

Make synchronous APIs take GCancellables, and make asynchronous APIs
use GAsyncReadyCallbacks and have names ending in "_async", with
"_finish" functions to retrieve the results.

Also, make nm_client_activate_connection_finish(),
nm_client_add_and_activate_finish(), and
nm_remote_settings_add_connection_finish() be (transfer full) rather
than (transfer none), because the refcounting semantics become
slightly confusing in some edge cases otherwise.