Add main menu with first "demo" entry, clean/improve/fix code

android/gradle.properties

assets/menu/demo.png

icons/build_application_icons.sh

icons/build_game_icons.sh

icons/build_icons.sh

icons/build_repository_icons.sh

icons/menu_demo.svg

lib/main.dart

lib/provider/data.dart

lib/screens/demo.dart

lib/screens/home.dart

pubspec.lock

pubspec.yaml