[Stub] Centralize validation outside of the control component

**Is your feature request related to a problem? Please describe.**

Field validation is implemented through multiple, somewhat conflicting methods that must be defined directly on the control component. If I want to do serious custom validation on a field with an existing widget, I can't - I have to make a new widget.

**Describe the solution you'd like**

Accept an `onValidate` method on the object passed to `registerWidget`. This will be difficult to do in a backwards compatible way, and should probably be brought in as a replacement for all current validation methods in the next major release.

**Additional context**

Will add issues that will be fixed/helped by this, there are quite a few.