Improve usability of Enclosure#add_screw

[raphael-isvelin]

It's a bit cumbersome to use, because it requires you to--before calling it--create a ScrewBlock only to get its size, to be able to set the correct position for the screw, as the screw is centred on (0,0). (Or to hardcode the size.)

A neat solution could be to create a new method like add_screw_on_side/line, accepting a side (e.g. UP, DOWN, LEFT, RIGHT), and a position on the line. This could, it could even determine automatically whether to use Z_TAPER_CORNER or Z_TAPER_SIDE.