Add a none-of-above option to the list

[mkhanal]

From a list of checkbox, there should be one, which when checked, unchecks all others and is unchecked when anything else is checked. This is so that a usual use case of "None of the above" option can be easily catered for. It would be good to have this allow both unsetting all the values or setting a special value for the none-of-above options

[beradrian]

These kind of cases can be easily handled by checklist-before-change or checklist-change.