add next_block method used by keyboard input
add next_block method used by keyboard input

walterbender/turtleart,walterbender/turtleconfusion,max630/turtleart-hacks,max630/turtleart-hacks,walterbender/turtle3D,walterbender/turtleflags,walterbender/AmazonasTortuga,walterbender/turtleartmini,AlanJAS/turtleart