What is the difference between WidgetsApp and MaterialApp?
WidgetsApp :
* A convenience class that wraps several widgets that are commonly required for an application.
* One of the primary roles that WidgetsApp provides is binding the system back button to pop the Navigator or quitting the application.
MaterialApp :
* A convenience widget that wraps several widgets that are commonly required for material design applications.
* It builds upon a WidgetsApp by adding material-design specific functionality, such as AnimatedTheme and GridPaper.
