Assign `menuitemcheckbox` role to all entries which can be toggled

[krassowski]

I think ideally the role for menu entries which can be toggled would always have menuitemcheckbox role, not only when they are toggled. I appreciate it is not trivial to achieve with the information available on menu items right now so it would require a larger change. I think this PR (#753) is a significant improvement as-is though.

Originally posted by @krassowski in #753 (review)