Post meta box toggles accessibility improvements

[afercia]

Thanks to the work done on #33544 the major accessibility issues are now fixed. There's still room for some improvements, most noticeably properly setting the aria-expanded initial attribute value on page load accordingly to their stored state. Without this aria-expanded can be set to true while the panel is actually closed.

[afercia]

First pass.

• set proper aria-expanded initial attribute value on page load
• toggle the buttons aria-expanded also when clicking on the headings
• avoid setting aria-expanded on the headings
• add type="button" attribute on the buttons, no need for preventDefault()
• remove the title attribute
• maybe better translatable string (paired with the ones used in the Customizer), open to suggestions
• fix for a ​Firefox + NVDA bug as already done in the Customizer
• completely hide the buttons when JS is off
• minor CSS adjustments

[joedolson]

'Toggle panel: %s' seems fine to me; it's functional and concise, which is all I think is particularly important.

I was unaware that adding 'type="button" would make preventDefault() irrelevant. That seems incredibly nonintuitive to me; but definitely good to know!

(Off to revise a whole ton of old code...)

[wonderboymusic]

If you're going to use $(this) a bunch of times, set it to $el once and then use that. Also, if you can, use $(e.target) instead, makes porting procedural code to methods easier.

[afercia]

Refreshed patch as per @wonderboymusic suggestion. Noticed $( this ) and $( event.target ) are different in this case (the buttons have a span inside) so used the $el approach. Except for the each() loop where maybe setting variables doesn't make much sense and $( this ) is more appropriate.

[wonderboymusic]

In 34893:

Meta Boxes: reboot some of the code in postbox.js to support aria-expanded attribute toggling and to properly reference static class properties.

Props afercia, wonderboymusic.
Fixes #33754.