No easy cross platform way to set or unset focus on a Button

There is no API or bead to set or unset focus on a Button.

You can do this in JS:
```
COMPILE::JS {
button.element.blur();
}
COMPILE::JS {
button.element.focus();
}
```

The issue provides no file or test entry point. Start from the Button API and the shown JavaScript button.element.focus() and blur() calls, then inspect how Button behavior is implemented across supported platforms. Done means a documented cross-platform way to set and unset Button focus, with coverage for the supported targets.

Written by the indexing model from the issue text.