core: extend nm_shutdown_wait_obj_*() to support notification via a GCancellable

Now nm_shutdown_wait_obj_*() supports two styles:

 - NM_SHUTDOWN_WAIT_TYPE_OBJECT: this just registers a weak pointer
   on a source GObject. As long as the object is not destroyed
   (and the object is not unregistered), the shutdown gets blocked.

 - now new is NM_SHUTDOWN_WAIT_TYPE_CANCELLABLE: this source object
   is a GCancellable, and during shutdown, the system will cancel
   the instances to notify about the shutdown. That aside, the GCancellable
   is tracked exactly like a regular NM_SHUTDOWN_WAIT_TYPE_OBJECT (meaning:
   a weak pointer is registered and shutdown gets delayed as long as the instance
   lives).

As the rest of the shutdown, it's not yet implemented on the shutdown-side.
What is now possible is to register such cancellables, so that users can make
use of this API before we fix shutdown. We cannot fix it all at the same time,
so first users must be ready for this approach.

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