[sailfish-browser] Add support for different kind of file pickers

Context of the component is wrong. Unfortunately Qt.createComponent doesn't provide parent argument.

Should be created in WebVIew.qml e.g. at the end as component.

```
    Component {
        id: pickerCreator
        PickerCreator {}
    }
```

And passed as a parameter to the WebPopupHandler.js when WebView gets completed.