Add form validation

[andydotxyz]

Adding Form.Validator to check the whole of a form and control if Submit is enabled.
(this is built on top of FormItem.Required to avoid conflicts)

Tests still to be included (- and balance the old entry validation specifics with this), PR is open for feedback just now, try this code:

// Package main loads a very basic Hello World graphical application.
package main

import (
	"errors"

	"fyne.io/fyne/v2/app"
	"fyne.io/fyne/v2/widget"
)

func main() {
	a := app.New()
	w := a.NewWindow("Hello")

	en1 := widget.NewEntry()
	en2 := widget.NewEntry()
	en2.Validator = func(in string) error {
		if in != "hi" {
			return errors.New("must be hi")
		}
		return nil
	}

	f := widget.NewForm(
		widget.NewFormItem("1", en1),
		widget.NewFormItem("2", en2))
	f.Validator = func() error {
		if en1.Text != en2.Text {
			return errors.New("en1 != en2")
		}
		return nil
	}
	f.OnSubmit = func() {}
	w.SetContent(f)

	w.ShowAndRun()
}

Fixes #2562

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.
• Public APIs match existing style and have Since: line.

[coveralls]

coverage: 60.442% (+0.04%) from 60.406% — andydotxyz:feature/formvalidate into fyne-io:develop