Problem: Button widget emit generic IP
Problem: Button widget emit generic IP

Solution: allow to specify an IP to emit when the button is clicked.
As the user will rarely want the `event` `'(button . #t)`, he can specify which
IP send when a button is clicked with the help of the option port :

```
(mesg "button" "option" '(a-random . ("ip" 'here 1)))
```

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