browser focusable behavior for pointer (mouse, touch)

[rodneyrehm]

The focusable data-tables currently only cover script focusable and keyboard focusable (tabbable). But focus can (obviously) also be shifted by clicking on things (pointer, mouse, touch). Of course script focusable and pointer focusable are not the same.

• The <legend> element can redirect focus to another element, but only on element.focus(), not on pointer focus. see demo
• The <label> element redirects focus to the referenced control element via element.focus() and pointer focus in Chrome and Firefox. In Internet Explorer focus is not redirected for script focus. see demo
• The only tabbable elements are also pointer focusable in Firefox and Internet Explorer.

We should add a third layer of testing to tests/focusable/test.html to cover pointer focus.