Allow focussing arbitrary components/elements

User Story

As an developer, I want to set focus to arbitrary page elements so that users with screen readers can be guided through a UI programmatically

Acceptance Criteria

e.g. allow focusing a label or a div

<zk>
    <label id="headline" tabindex="-1" value="focus me" />
    <button onClick="headline.focus()"/>
</zk>

Details

currently 'not all' widgets support focus (without a specific list)
https://www.zkoss.org/javadoc/latest/jsdoc/zk/AuCmd1.html#focus-zk.Widget-

In general any component can have a tabindex so they should also be focusable