Add minimal gameplay

android/gradle.properties

assets/icons/button_back.png

assets/icons/button_start.png

assets/icons/difficulty_easy.png

assets/icons/difficulty_hard.png

assets/icons/difficulty_medium.png

assets/icons/difficulty_nightmare.png

assets/icons/game_fail.png

assets/icons/game_win.png

assets/icons/size_10x10.png

assets/icons/size_15x15.png

assets/icons/size_20x20.png

assets/skins/default_0.png

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