GObject signals only complicate the code and are less efficient.

Also, NM_DEVICE_AUTH_REQUEST signal really invoked an asynchronous
request. Of course, fundamentally emitting a signal *is* the same as
calling a method. However, implementing this as signal is really not
nice nor best practice. For one, there is a (negligible) overhead emitting
a GObject signal. But what is worse, GObject signals are not as strongly
typed and make it harder to understand what happens.

The signal had the appearance of providing some special decoupling of
NMDevice and NMManager. Of course, in practice, they were not more
decoupled (both forms are the same in nature), but it was harder to
understand how they work together.

Add and call a method nm_manager_device_auth_request() instead. This
has the notion of invoking an asynchronous method. Also, never invoke
the callback synchronously and provide a cancellable. Like every asynchronous
operation, it *must* be cancellable, and callers should make sure to
provide a mechanism to abort.

(cherry picked from commit b5070277)