Add simple screen

Add 'Simple screen' to template, but give it a better name like 'BaseScreen' for example. Replace every screen in the app with this screen, including the debug screens. Add an automatic back button if the current route can go back so this doesn't have to be passed everytime. You can check if the current route can be popped by using: 
`ModalRoute.of(context)?.impliesAppBarDismissal`