Use the Sage displayhook, otherwise graphics is not shown (trac #14469)

repr() of a graphics object used to display the graphics as a side-effect. I stopped this insanity at http://trac.sagemath.org/14469. But now the notebook needs to be smarter about when to call show(). The easiest solution is to just use the displayhook that we are also using for doctesting (but without DOCTEST_MODE, of course).