f:ajax - add onprogress attribute to let the dev pass a js callback

This is a missing part of the javascript ajax engine of Faces

to be on par with the standard XMLHttpRequest

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequestEventTarget/progress_event

```

```

```
public class AjaxBehavior extends ClientBehaviorBase {

enum PropertyKeys {
// ...
onprogress
}

public String getOnprogress() {
return (String) getStateHelper().eval(PropertyKeys.onprogress);
}

public void setOnprogress(String onprogress) {
getStateHelper().put(PropertyKeys.onprogress, onprogress);
}
}
```

Start with the AjaxBehavior class shown in the issue and trace the JavaScript AJAX engine that handles f:ajax attributes. Compare its event handling with the referenced XMLHttpRequest progress event. Done means the onprogress attribute is accepted and the supplied callback receives progress data during the request.

Written by the indexing model from the issue text.