Hiding implicit focus indicators about huh

[dez11de]

Creators of such forms that don't want the little bar can simply set theme.Focused.Base.BorderStyle to HiddenBorder.

[meowgorithm]

From a design perspective I'd prefer to default to hiding implicit indicators to reduce visual noise.

There's also a catch: focus indicators may not necessarily be borders: a theme could instead employ something different, such as a background color on the group, so manually adjusting borders won't necessarily do the trick.

If we find the former behavior is desired we could also add an option to enable it.