Add/improve skins

assets/skins/retro_2.png

assets/skins/retro_3.png

assets/skins/retro_4.png

assets/skins/retro_5.png

assets/skins/retro_6.png

assets/skins/retro_7.png

assets/skins/retro_8.png

assets/skins/symbols_0.png

assets/skins/symbols_1.png

assets/skins/symbols_2.png

assets/skins/symbols_3.png

assets/skins/symbols_4.png

assets/skins/symbols_5.png

assets/skins/symbols_6.png

assets/skins/symbols_7.png

assets/skins/symbols_8.png

icons/build_game_icons.sh

icons/level_easy.svg

icons/level_hard.svg

icons/level_medium.svg