Add a stop button

[cameronfreer]

Some queries (e.g., approximate mutual information calculations in big crosscat models) are valid (hence do not lead to an immediate error) but are slow, hence run for a very long time. One can reclaim the ability to enter queries (at expense of losing one's unsaved work) by reloading the adoc website in the browser, but it would be even better if one could hit a "Stop" button (either globally at the top, say; or next to each "Execute" button; or next to the currently executing cell's "Execute" button).