Widget state attributes are not updated when widget updates

[lhengl]

Hi first off, great package.

However, what I noticed is that when I change attributes in the widget, it is not reflected in the UI. This is because the attributes are stored in the state object during initState, so doing a rebuild of the IntroSlider widget will not update the UI. This is not a great user experience when trying to set up the widget for the first time using hot reload to see the changes in the UI.

May I suggest we add a didWidgetUpdate() in the state class or using the widget attributes directly rather than saving them as a state variable? What I noticed is that they are mostly defined as late final anyway, why not just refer to them as widget.attribute?

The workaround has been to use a UniqueKey() to force the widget to re-initialise itself on each refresh - however I feel that this must not be very efficient in the case where we need to change the widget attribute frequently (an example I could think of is when we need to enable/disable navigation based on some validation rules as user inputs data).