Use the new modern :focus-visible instead of :focus. This is only shown
when navigating using the keyboard.
And in the case of the control bar buttons, This means we can separate
the :focus and :hover styles. Instead of showing a lighter overlay (or
darker for selected) like we use for hover, lets use a more common
blue outline for focus-visible. This also means we can re-use the common
focus-visible from input.css instead of having a special one for control
bar buttons.
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