Improve game architecture

android/app/build.gradle

android/gradle.properties

assets/fonts/Nunito-Bold.ttf

assets/fonts/Nunito-Light.ttf

assets/fonts/Nunito-Medium.ttf

assets/fonts/Nunito-Regular.ttf

assets/icons/button_delete_saved_game.png

assets/icons/button_resume_game.png

assets/icons/level_easy.png

assets/icons/level_hard.png

assets/icons/level_medium.png

assets/icons/level_nightmare.png

assets/icons/size_2x2.png

assets/icons/size_3x2.png

assets/icons/size_3x3.png

assets/icons/size_4x4.png

assets/icons/skin_digits.png

assets/skins/digits_1.png

assets/skins/digits_10.png

assets/skins/digits_11.png