Radio/Checkbox lists with "Other" option needed

[jace]

RadioField and SelectField (with Checkbox widget) currently do not support an "Other" option. This is often required and should be fairly easy to implement:

1. Provide alternate widgets that append a text field with an "Other" label at the bottom of the list, using the same name value (with '-other' suffixed) so that the contents of the field are submitted.
2. Use a sensible placeholder value in the radio/checkbox widget on the form to indicate to the field that the name-other field's value should be read. Perhaps just a blank string as anything else could potentially conflict with existing choices.
3. Provide alternate fields that don't strictly validate against the choices list and look in name-other for values.