game elements have now top, bottom, left, right, center_x and center_y properties
fixed label assignations
Ball: changed (set/get)_speed_(x/y) and  to speed_(x/y) properties
moved wall collision to common file
Bricks can collide to balls- but not yet bounce it.
added "are_colliding_rect_rect(elem1,elem2)"
replaced "modify_heading(heading)" with heading property