Instead of setting multiple callbacks, just let the user set one
vtable with callbacks. Usually, GObject would implement this via
signals. While that makes sense for public objects, for example to
work better with GIR and allow intercepting the signal, this is
overkill for our internal type. And NMPolkitListener already did
not make use of signals, for good reason.
Instead of passing multiple callbacks, must pass one structure with
callback pointers.
Also, extend the signature of the callbacks to always contain a
@self argument and a @user_data.