GUI single stepper

Using the old listener-based single stepper as a starting point, I implemented a much improved graphical tool for walking through code. It is easier to use than the old single stepper, automatically shows the stacks at each point, supports a new 'step out' action which executes until the end of the current quotation, and has better support for stepping through code which throws errors. This last part is still pretty broken, however. Also stepping through code which works with continuations doesn't do the right thing at all. I plan on fixing both issues before 0.84.