Pause button for reactors

[jpolitz]

It would be useful for reactors (in CPO) to have a built-in pause button, or even a configuration option called pause-key.

It's a lot to implement pause on your own, and it's super useful.

[blerner]

Ideas: have a button in the title bar of the reactor window. Also allow a "pause-key" attribute on reactors for keyboard toggling. Finally, add a "paused-image" optional handler, to go along with "final-image", that draws the screen when the reactor is paused.