Add a signal on date selected

[Nicolas Laplante]

We should be able to connect a signal handler on the date picker when the date is selected / changed in the calendar. Right now we have to use something like this:

var myDatePicker = new Granite.Widgets.DatePicker ();
myDatePicker.notify["date"].connect ((s, p) => {
    // ...
});

IMHO it should be doable like this:

myDatePicker.date_changed.connect ( ... )