Adding the class Point, removing the attributes x and y from

Adding the class Point, removing the attributes x and y from
the Ball class and adding the p attr that is an instance of Point.
Updating BallManager.save_to_file, Gzv.draw_ball to fit with the above
change.

The feature "Move ball" was implemented and to do so, the event
motion-notify-event of the Gzv.treeview was binded to a new
callback (Gzv.ball_motion), Gzv.button_pressed now discovers if
the click was inside an already existent ball and were added
two new attributes to Gzv class: last_y and move_ball.