Add validation to Multiple field settings

This turns the two Multiple values settings, that we assume to be numbers, into numbers fields.

Tested this:

these will eventually get fixed once they're resaved from the UI and the validation error kicks in

Turns out this isn't what happens if you just make these number fields. If you start with a string value and then apply this fix and reload the View it will show an empty value. So no validation error is given if we solve it like this.
We could validate this with actual form validation rules and in that case it would throw the error. But that feels a little overkill to me, also since we already hardend the ode against people sneaking in strings

Opinions welcome!

I agree that it makes sense change to a number form.

Maybe a follow up could be open to add an "all" option

Form could maybe be

All - new
Custom - current setup of using Display N value(s)

Question though will an update hook be needed to transform current values from string to int or is the form API smart enough to know that.