[A11Y] Make checkboxes focusable (#3014)

* Add extra feature to a11y focusring mixin

* Add visually hidden CSS class and mixin

* Visually hide checkboxes (keep in focus/a11y tree)

* Place checkbox focus ring around display element

* Improve mobile checkbox/switch accessibility

framework/core/js/src/common/components/Checkbox.js

@@ -33,7 +33,9 @@ export default class Checkbox extends Component {
     return (
       <label className={className}>
         <input type="checkbox" checked={this.attrs.state} disabled={this.attrs.disabled} onchange={withAttr('checked', this.onchange.bind(this))} />
-        <div className="Checkbox-display">{this.getDisplay()}</div>
+        <div className="Checkbox-display" aria-hidden="true">
+          {this.getDisplay()}
+        </div>
         {vnode.children}
       </label>
     );