Overhaul moving and jumping; add basic gravity

Allow for long key presses for walking; move on each render, not on each
keypress (which makes the motion choppy).

Gravity is a very basic and linear velocity change. Probably should make
more realistic.

Happy that, now, can walk and jump at same time.

Walking is still buggy because does not gracefully handle when a
different arrow is pressed at same time.

Touch completely broken.