'Add a link to response.s3.actions
@param label: the link label
@param url: the target URL
@param attr: attributes for the link (default: {"_class":"action-btn"})'

@staticmethod
def action_button(label, url, icon=None, **attr):

    link = dict(attr)
    link['label'] = s3_str(label)
    link['url'] = url
    if (icon and current.deployment_settings.get_ui_use_button_icons()):
        link['icon'] = ICON.css_class(icon)
    if ('_class' not in link):
        link['_class'] = 'action-btn'
    s3 = current.response.s3
    if (s3.actions is None):
        s3.actions = [link]
    else:
        s3.actions.append(link)
