Settings button: Firefox and NVDA don't announce the aria-expanded state

[afercia]

See #1387, #1388, #1537, #1560.

Although the original NVDA bug was fixed in the 2018.4 alpha builds, the fix was then reverted. Thus, there are still cases where Firefox and NVDA aren't able to announce a change in the aria-expanded state.

As explained in the NVDA issue, this is caused by a mix of reasons:

• when there's a CSS pseudo-element change, Firefox rebuilds the set of accessible properties (Firefox bug)
• NVDA thinks it's a new element and doesn't speak anything, not even a focus change (NVDA original bug)

In the case of the Settings button, when the button is not toggled, there's no CSS pseudo-element. Only when the button is toggled, a CSS pseudo-element gets generated.

Testing with Firefox 65 and NVDA 2018.4.1 nothing is announced. See in the animated GIF below:

• when landing on the button, NVDA correctly announces "Settings button expanded"
• toggling the button, nothing gets announced

Seems to me this button styling can be implemented even without the CSS pseudo-element, which would solve the issue. Will try a quick PR.