This guards function calls by storing the function, args and kwargs into
an added attribute '__auth_fcall' on the object using the decorator,
then emits a signal that can be handled by the UI.
The UI can signal auth-success or auth-failure back to the object by
calling either the authProceed() slot or the authCancel slot.
The object utilizing this decorator MUST inherit/mixin the AuthMixin
class, which provides the above two slots, and handling of state.
The decorator also accepts a 'reject' parameter, containing the name of
a parameterless function on the object, which is called when
authentication has failed/is cancelled.
Sander van Grieken