Menu Customizer: available menu items (and widgets) after preview iframe should be really hidden and out of tab order

[afercia]

The available Menu Items panel (and also the available Widgets panel) are placed in the HTML source after the preview iframe. They're hidden because they're just out of view but still focusable.

To reproduce, navigate the Customizer using your keyboard and tab through all the elements in the preview until you reach the last focusable element, e.g. like in the screenshot below:

Now keep tabbing and you will notice there are several tab stops before you can actually reach the end of the page. You're actually navigating through all the available Widgets and the available Menu Items controls.

Screen readers will announce all this content since it's actually "visible" for assistive technologies.

The Customizer is already using visibility: hidden to hide hidden elements from keyboard focus and screen readers, see 33069 and that should be applied here too.

[valendesigns]

@afercia I'll test this patch for you tomorrow :)

[ocean90]

In 33136:

Customizer: Use visibility to hide available widgets and menu items panels from screen readers and to move them out of tab order.

props afercia.
fixes #32883.