Add "nature" skin

android/gradle.properties

assets/icons/skin_nature.png

assets/skins/nature_1.png

assets/skins/nature_2.png

assets/skins/nature_3.png

assets/skins/nature_4.png

assets/skins/nature_5.png

assets/skins/nature_6.png

assets/skins/nature_7.png

assets/skins/nature_8.png

assets/skins/nature_9.png

icons/build_game_icons.sh

icons/skin_nature.svg

icons/skins/nature/1.svg

icons/skins/nature/2.svg

icons/skins/nature/3.svg

icons/skins/nature/4.svg

icons/skins/nature/5.svg

icons/skins/nature/6.svg

icons/skins/nature/7.svg