Scrollable checkboxes should be an option of layout for checkboxes

[ebruchez]

This could just be an extra appearance in addition to full, so full scrollable (or full xxf:scrollable). In the UI, this would be a checkbox appearing when the full (radios/checkboxes) appearance is selected.

The scrolling could be implemented directly in CSS with the class representing the appearance, e.g. xxforms-appearance-scrollable.

When this is done, the fr:box-select component could be deprecated and FB should convert that to the appearance.

See also #333 for the horizontal/vertical/column layouts.