a11y: Introduce a dedicated role for toggle buttons

a11y: Introduce a dedicated role for toggle buttons

Up until now, toggle buttons were presented as regular push buttons.
That's the approach used by the ARIA specification, however, our platform
accessibility backend, at-spi2, can not represent accessibe states with values,
so we can not represent the design pattern precisely enough for screen readers.
If, in future, the a11y backends gain this capability, we might consider again
removing this role.