Return a Button widget with specified `handler`.

def make_button_widget(cls, label, file_path=None, handler=None, style=None, layout=Layout(width='auto')):
        "Return a Button widget with specified `handler`."
        btn = widgets.Button(description=label, layout=layout)
        if handler is not None: btn.on_click(handler)
        if style is not None: btn.button_style = style
        btn.file_path = file_path
        btn.flagged_for_delete = False
        return btn