Use the GCancellable to cancel pending operations during destruction

Relying on references to stay alive across idle and asynchronous calls
confuses users because the object stays alive even after the user has
dropped the only reference known to it. This can lead to crashes due
to invalid memory access if the object or its children emit signals
after the user has itself been destructed.

https://bugzilla.gnome.org/show_bug.cgi?id=741257