bug(web): (re-)allow to set default keyboard to 'off'

Older Keyman versions allowed to set the default keyboard for a field to 'off', i.e. the system keyboard. Clicking in the field wouldn't show the OSK but would still show the Keyman button and allow to turn on a Keyman keyboard. See older versions of the Control-by-Control sample.

The way to do it was by passing the empty string for the keyboard and languageCode parameters:

```
keyman.setKeyboardForControl(document.f.subject, '', '');
```

With current Keyman versions this is no longer possible; the first keyboard added will be the default keyboard. This was always the behavior on touch-based devices.

It would be nice to re-implement this functionality.