ReactiveCommand async registration and subscription

reactivecommand async registration and subscription

from(enable->RegisterAsyncFunction(
    [](RoutedEventPattern)
    {
        // background thread
        // enable and disable commands will be disabled until this is finished

        std::this_thread::sleep_for(std::chrono::seconds(2));

        return true;
    })) // this is a subscription to the enable ReactiveCommand
    // back on the ui thread
    .subscribe([this](bool) // takes whatever was returned above
    {
        // update the ui
    });