use lambda, a new feature of C++11, for event answering

Nana-int-04.cpp

fm.make_event<events::click>([]{
        //When the form is clicked, the object created
        //by this lambda will be “called”.
});

//or

fm.make_event<events::click>([](const eventinfo& ei){
        //When the form is clicked, the object created
        //by this lambda will be “called”, and I can
        //read the parameter.
});