Feathers checkbox and slider should auto-update

## Discussion

When creating an interface with feathers there are a couple of gotchas because of the `controlled` nature of the feathers architecture. However other controls (for example button, number input) entirely self manage their linking of display and underlying state.

I can see good reasons why self update might not always be wanted (which is the intention of leaving them unlinked), but by default it should be called itself. I would suggest that a property be added to both controls `self_update` which when true (default) automatically adds:

on(checkbox_self_update)

or

on(slider_self_update)

## What you currently need for a checkbox

```
@FeathersCheckbox {
@caption: bsn! {
caption("Checkbox")
}
}
on(checkbox_self_update) // <<< without this the control silently fails to operate
```

## What you would need after

```
@FeathersCheckbox {
@caption: bsn! {
caption("Checkbox")
}
}
```

## How you would achieve the current behaviour

```
@FeathersCheckbox {
@caption: bsn! {
caption("Checkbox")
}
@self_update: false,
}
```

(same examples for slider)

Note - this would be a breaking change requiring upgrade but I think it is worth it to set up feathers for usability.

## Other Controls that this same change would be good for

- radio (radio_self_update added to group not to individuals)
- list box (listbox_update_selection)
- toggle (uses checkbox_self_update)

Start by locating the FeathersCheckbox and slider controls and tracing the existing checkbox_self_update and slider_self_update behavior. Compare them with the self-managed button and number input controls, then verify that the proposed default behavior works while the self_update false path preserves current behavior for checkbox and slider, including the related radio, list box, and toggle cases.

Written by the indexing model from the issue text.