Added didUpdateWidget. This function called whenever the widget configuration changes.

[Husnain-Asharf]

If the parent widget rebuilds , the framework will update the [widget] property of this [State] object to refer to the new widget and then call this method with the previous widget as an argument.

Override this method to respond when the [widget] changes.

The framework always calls [build] after calling [didUpdateWidget], which means setState not required in thi function.

I used this function update current date if it's change and animate to this date.