Checkbox to select all the fields

Hello,
 
We suggest using the fieldslistopen event to track the Field List opening. Next, add the corresponding button that would iterate over fields and select unselected ones.
You are welcome to see the JSFiddle we have prepared for the demonstration: https://jsfiddle.net/flexmonster/q401gzyd/.
 
Please note that this JSFiddle only serves as a reference. It may require further improvements to fit your needs.
 
We hope it works well for you.
 
Regards,
Illia