Make menubar accessible

[paulkaplan]

According to https://www.w3.org/TR/2017/NOTE-wai-aria-practices-1.1-20171214/examples/menubar/menubar-1/menubar-1.html

Currently it is not.

[paulkaplan]

I'm gonna take mini off this one, I think it needs-discussion with someone who has more knowledge in this area to figure out exactly what we want. I don't think "per the aria spec" is specific enough for scratch, where we have some pretty complex interactions between keyboard controls/accessibility.