Clean / improve / update code

android/gradle.properties

assets/ui/button_back.png

assets/ui/button_delete_saved_game.png

assets/ui/button_resume_game.png

assets/ui/button_start.png

fastlane/metadata/android/en-US/changelogs/29.txt

fastlane/metadata/android/fr-FR/changelogs/29.txt

lib/config/default_game_settings.dart

lib/config/default_global_settings.dart

lib/config/menu.dart

lib/ui/game/game_end.dart

lib/ui/helpers/styled_button.dart

lib/ui/layouts/parameters_layout.dart

lib/ui/parameters/parameter_image.dart

lib/ui/parameters/parameter_painter.dart

lib/ui/parameters/parameter_widget.dart

lib/ui/widgets/actions/button_delete_saved_game.dart

lib/ui/widgets/actions/button_game_quit.dart

lib/ui/widgets/actions/button_game_start_new.dart

lib/ui/widgets/actions/button_resume_saved_game.dart