Warn before quitting while an image/batch is generating

[Leland]

It can feel pretty painful to accidentally quit during a 75 step image. It'd be great if there was a confirmation dialogue that appeared during an active batch/image to prevent that.

There should be no dialogue shown while the app is idle, of course.

This should be pretty straightforward, with a window.onbeforeunload event, and then toggling behavior via is_input_avail status.