Preview addon: Better update delay logic.

While typing, update very 1000ms (slow), but after stopping typing, update after 150ms (fast). Then, while typing, the preview is updated only slowly, just so that one sees that everything is working from the corner of one's eye. But when stopping typing, the preview is updated quickly so that the preview has the newest version quickly.

This should roughly match what was suggested in the source code comment as a possible improvement.