Fix the update time step

Snake's speed is fixed by a constant
If the user press a key the update cycle is launch to manage each change
of direction
This must be restricted only to arrow key in a further patch
Simplifies the management of control evenement

Now the new direction is set by a dictionnary
Escape key now close the application
Only arrow key can update the snake

If the new direction is the opposite of the actual direction
the direction is not change and there is no snake's update