Add minimal home page, settings and gameplay

icons/build_application_icons.sh

icons/build_game_icons.sh

icons/button_start.svg

icons/difficulty_easy.svg

icons/difficulty_hard.svg

icons/difficulty_medium.svg

icons/size_2.svg

icons/size_3.svg

icons/skin_default.svg

icons/skins/default/1.svg

icons/skins/default/2.svg

icons/skins/default/3.svg

icons/skins/default/4.svg

icons/skins/default/5.svg

icons/skins/default/6.svg

icons/skins/default/7.svg

icons/skins/default/8.svg

icons/skins/default/9.svg

icons/skins/empty.svg

lib/entities/cell.dart