"toggle" doesn't seem like quite the right name because it doesn't toggle anything--calling it multiple times with the same arguments won't cycle between states. [I pointed this out to Evelyn](https://github.com/mozilla-b2g/gaia/pull/12092#discussion_r6300635), and she created [string-accepting `showButtons` and `hideButtons` methods](https://github.com/mozilla-b2g/gaia/blob/3f51f302c3a0c57d8bad482ec7ee86b2819389fb/apps/clock/js/stopwatch_panel.js#L60-L70). What do you think of this refactoring?