Clean / improve / update code

android/gradle.properties

assets/skins/default_1.png

assets/skins/default_2.png

assets/skins/default_3.png

assets/skins/default_4.png

assets/skins/default_5.png

assets/skins/default_6.png

assets/skins/default_7.png

assets/skins/default_8.png

assets/skins/default_found.png

assets/skins/default_unknown.png

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/26.txt

fastlane/metadata/android/fr-FR/changelogs/26.txt

lib/config/default_game_settings.dart

lib/config/default_global_settings.dart

lib/ui/helpers/styled_button.dart