[Proposal] Add topmost className prop and merge it into trigger element

[yenshih]

Due to this issue: https://github.com/ant-design/ant-design/issues/10325, I suggest that we add className as a prop of Trigger for convenience.

Current

const props = this.props;
const children = props.children;
const child = React.Children.only(children);
const newChildProps = { key: 'trigger' };

// Merge some event handlers into `newChildProps`

const trigger = React.cloneElement(child, newChildProps);

Expected

import classNames from 'classnames';

// ...

const props = this.props;
const children = props.children;
const child = React.Children.only(children);
const newChildProps = {
    key: 'trigger',
    className: classNames(props.className, child.props.className),
};

// Merge some event handlers into `newChildProps`

const trigger = React.cloneElement(child, newChildProps);

name	type	default	description
className	string		additional className added to trigger

@yesmeck Shall I create a PR directly? Thanks.

[yenshih]

ping~ @afc163 @yiminghe @yesmeck