Performance improvement for determinate progress indicators: poll the atom instead of watching it

[pmonks]

Currently, a determinate progress indicator adds a watch on the caller's "progress" atom, which can result in excessive redrawing behaviour if that atom is updated at a high rate (say thousands of times per second).

A better approach would be to simply poll the atom on a (configurable) fixed schedule (say, every 100ms by default), and only redraw if the value has changed.