Add header.png and footer.png support for games, and support texture packs via <gameid>_menu_<background/overlay/header/footer>.png

Also clean up the_game a bit: move node placement prediction to a
separate function.

Add a Way of checking for specific Feature with Lua Adds minetest.get_feature() and minetest.has_feature()

Check in all 6 directions instead of just 3.