Link line brushing

When this is on, then two or more XGobis that were initiated with
the same data (or have the same number of rows) *or* have the same
value of `fname.nlinkable' *and* have the same number of connected
lines are linked for line brushing.  That is, line brushing actions
performed in one of the XGobis is promptly reflected in the
others.

This is most frequently used when the XGobi plotting windows
display different views of the same data.

Note:  For computational efficiency, the receiving XGobis should be
in XYPlot, Plot1D, or paused in Rotate or Tour.

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

*linkLineBrush:   True