Improve title bar

android/gradle.properties

assets/game/difficulty_3x3.png

assets/game/difficulty_4x4.png

assets/game/difficulty_5x5.png

assets/icons/button_back.png

assets/icons/button_shuffle.png

assets/icons/difficulty_3x3.png

assets/icons/difficulty_4x4.png

assets/icons/difficulty_5x5.png

icons/build_application_icons.sh

icons/build_game_icons.sh

icons/button_back.svg

icons/button_shuffle.svg

icons/difficulty_3x3.svg

icons/difficulty_4x4.svg

icons/difficulty_5x5.svg

lib/screens/home.dart

pubspec.yaml