Upgrade flutter framework and dependencies, clean/update code

assets/skins/nature_9.png

fastlane/metadata/android/en-US/changelogs/56.txt

fastlane/metadata/android/en-US/images/icon.png

fastlane/metadata/android/fr-FR/changelogs/56.txt

icons/build_application_icons.sh

icons/build_game_icons.sh

icons/cell_empty.svg

icons/featureGraphic.svg

icons/level_easy.svg

icons/level_hard.svg

icons/level_medium.svg

icons/level_nightmare.svg

icons/placeholder.svg

icons/skins/food/10.svg

lib/entities/cell.dart

lib/layout/board.dart

lib/layout/game.dart

lib/layout/parameters.dart

lib/provider/data.dart

lib/screens/home.dart