Update brushing continuously

If this button is on, then brushing updates are performed
continously:  that is, whenever a brushing action is performed in
the active window, any linked windows are updated immediately.

If it is off, then the updates only occur when you lift your
finger off the mouse button.  This helps make it possible to
perform linked brushing on data with many thousands of cases.

This can be set in the XGobi resource file.  The default is

*syncBrush:   True