title: onResume() and onPause() for widgets on Flutter
body: <p>Right now, a widget only has initeState() that gets triggered the very first time a widget is created, and dispose(), which gets triggered when the widget is destroyed. Is there a method to detect when a widget comes back to the foreground? and when a widget is about to go to the background because another widget just was foregrounded?
  It's the equivalent of onResume and onPause being triggered for Android, and viewWillAppear and viewWillDisappear for ios</p>