#30 Working/Cancel button

[Tom Downey]

Create a way for objects to run compute-intensive work in a separate thread. Have some way in the GUI to indicate that background computation is taking place ("Working..."). Have some way in the GUI to kill the computation. Be mindful of Swing non-threadsafeness. Maybe use the producer/consumer queue class to put computations onto a queue for a compute thread to run, and computations watch a boolean to see if they should abort (which is the new preferred way to stop a thread, instead of using stop()). Mostly effects certain Drawables that have big loops (Graph1D, etc.) or where each run through the loop can be expensive (RiemannSumRects with volume shapes). Would also help out with the Taylor polynomial applet when the degree is larger than 7 and the function being expanded has derivatives that get more complex (e.g., tan).