Add "food" skin, improve skins

assets/skins/nature_3.png

assets/skins/nature_4.png

assets/skins/nature_9.png

icons/build_game_icons.sh

icons/skin_food.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/food/1.svg

icons/skins/food/2.svg

icons/skins/food/3.svg

icons/skins/food/4.svg

icons/skins/food/5.svg

icons/skins/food/6.svg