Add a button to stop all the jobs initiated by Cascade

[reprogrammer]

Let's say I've started Cascade and it's computing its tree. While the tree is being computed, I decide to change the source code. I may no longer trust the results of the ongoing tree computation. In this case, I'd like to stop all the running jobs and compute the tree from scratch. It would be nice to have a "stop" button (e.g., a red square) that would stop all the running jobs of Cascade.