Add new option for "(Un-)Select all"

Thanks @cbccharlie I agree this would be super helpful!

The MR needs a reroll.

Just realized, I really don't like the approach, creating the checkbox through the javascript, we should create it inside the widgets "formElement" instead and also use two buttons "Unselect all" and "Select all" instead of one checkbox.

Ok, nevermind, instead links should be used AND the selection should display on every level.

Never mind, let's not change the existing functionality too much. Using links instead of the checkbox and adding them on each term level, can be done in a follow-up issue!

Please review! The branch aimed at 2.0.x is basically the implementation by @cbccharlie with some minor adjustments! There are two screenshots of the form attached.

Thanks @Grevil! I think we should now have @thomas.frobieter have a look at the classes and details. Did you test it works as expected and the additional checkbox doesn't interfer with the form checkboxes?

Did you test it works as expected and the additional checkbox doesn't interfere with the form checkboxes?

Yes, it works as expected, as you can see from the screenshots! No, it doesn't interfere with the form checkboxes! :)