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    supplicant: chain asynchronous requests for WPS
    
    Don't have pending asynchronous requests in parallel, like setting
    "ProcessCredentials" and "Start", or "Cancel" and "Start".
    Instead, "Start" is only scheduled after "ProcessCredentials" completed
    and "ProcessCredentials" is only scheduled after "Cancel" completed.
    
    Also, handle the async response of these requests. For one, to achive the
    chaining mentioned above and to log what happens and possible errors.
    
    Upon new enrollment, a previously created GDBusProxy is now reused,
    where the first operation is to Cancel the previous action.
    
    Also, consistently <trace> log what is happening.
    
    Not doing all of this is less lines of code. It's also simpler, and
    faster. But in my opinion, it is (usually) better to check and wait for
    return values, instead of firing off async requests uncontrolled. It
    allows us to better know where we are and to log about each individual
    step. This also makes all operations cancellable.
    Undoubtedly, correctness and handling failures conflicts with simplicity
    in this case -- or at least: what I think is "correctness" conflicts.