Fix multiple problem with listeners for contour groups. * scripts/plot/contour.m: Add new %!demo showing use of a single contour line. * scripts/plot/private/__contour__.m: Fix bug with lowest level of contourf showing background through. Use same routines for calculating levels and levelstep in main code and in listeners. Fix linecolor when turning "fill" on and off. Fix "textlist" and "textliststep" properties which were not activating correctly.