add callback for text show/hide

[pchampio]

Those callback are useful when wanting to display the OS virtual keyboard.

fixes #274

Linux example:

package main

// the file virtual_keyboard_linux.go is used to show the 'onboard'
// vitrual keyboard upon text input.

import (
	"os/exec"
	flutter "github.com/go-flutter-desktop/go-flutter"
)

var onBoard = exec.Command("onboard")

func init() {
	options = append(options,
		flutter.VirtualKeyboardShow(func(){
			err := onBoard.Start()
			if err != nil {
				return
			}
		}),
	)
}

[pchampio]

I plan to tag this into the next minor (or major) release.