Add option to validate form inputs

[pklaschka]

Add an option to validate form inputs and disallow submitting the form when the input is not valid.

One idea for this would be a new callback that gets passed with the options. This would (getting passed the form values) – e.g. – return validation error messages if the form is invalid or something like null to indicate the form is valid and the submission can "continue".

[pklaschka]

The question is where validation error messages could get displayed. If anyone has suggestions, please feel free to mention them here 😉

[pklaschka]

For now, I'll just implement this so that it will disable or enable the ok button, a more "complex" behavior might get implemented later...