Data structure that can be used to build the UI (only use it if it is helpful. Might be easier to just hardcode all in the template). Make sure to prefix the name of any input created here, so the names don't clash with real fields in the form.
Save the resulting selection as a space seperated list of css classes into the textarea with name={{ name }}. It expects something like:
hidden-xs visible-sm-inline-block visible-lg-block
In the example "None" was selected for md, so there is no entry for it. I don't do any server side validation, so please make sure there are no conflicting entries.
{{ widget_html }}
Other Template variables:
    id: {{ id }} (css id of the textarea)
    name: {{ name }}
    value: {{ value }}