*Please note:* I don't really know anything about a11y, hence it's possible that this patch either doesn't work correctly or at least isn't a complete solution.
In both the SecondaryToolbar and the Sidebar we have "button groups" that functionally acts essentially like radio-buttons. Based on skimming through [this MDN article](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/radio_role) it thus appears that we should tag them as such, using `role="radiogroup"` and `role="radio"`, and then utilize the `aria-checked` attribute to indicate to a11y software which button is currently active.
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