Add/improve skins

icons/level_nightmare.svg

icons/level_normal.svg

icons/skin_default.svg

icons/skin_images.svg

icons/skin_retro.svg

icons/skin_symbols.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/retro/0.svg

icons/skins/retro/1.svg

icons/skins/retro/2.svg

icons/skins/retro/3.svg

icons/skins/retro/4.svg

icons/skins/retro/5.svg