Is your feature request related to a problem? Please describe:

Sometimes validating each entry of a form on its own isn't enough.

Imagine the following scenario:
A form with two entries for numbers and the requirement that those numbers added up can't be above ten.

Is it possible to construct a solution with the existing API?

You can work around the issue by adding the validator mentioned above to all relevant entries. This is especially difficult if those entries already have validators on their own. For example if the first number can't be two and the second one needs to be above four.

Describe the solution you'd like to see:

A form wide validator would solve the problem. It would be of the type func() error and be evaluated every time one entry, checkbox etc. is changed.

If the function returns an error it should prevent the user from submitting the form (by disabling the submit button) and displaying the error message in red below the form similar to how an error on validating an entry is displayed.