The label "Show OK checkbox" is temporary wording. Amelia is looking into better wording. I went with `appearanceControl` for consistency with the existing `readOnlyControl`, but I have no problem changing the name if there's a problem with `appearanceControl` (for example, if it conflicts with some other use of the same name or whatever).

I think there's a larger design issue in the list of names in `widgets.js:getMainProperties()`, which looks like tight coupling between that function and the control element definitions in mugs with "special" needs (e.g., `AndroidIntent`). I decided to go with the more generic `appearanceControl` since that name is applied to every mug by `getMainProperties`, and other mugs may want to allow customizing their appearance as well. I decided not to tackle fixing this larger design issue in this round since it seemed like it might greatly increase the scope of what I am doing here.