This change adds a turtle method movexy for relative cartesian motion.

The reason to add the method is to simplify an introductory lesson on subclassing.

In http://activity.pencilcode.net.dev/edit/worksheet/classes.html, we ask students to try subclassing Turtle by adding velocity and spin properties, and a drift() method.  Without @movexy, the drift method must do a bunch of verbose vector arithmetic to apply the velocity, which is not the point of the lesson.  With @movexy, it is nice and concise.