topcat: fix bug relating to BoxLayout

Peter reported a stacktrace "java.awt.AWTError: BoxLayout can't be shared"
at a Container.removeAll call, java6 but not java5.  I can't really see
what it was complaining about, but rewording the code seems to have
sorted it out.

topcat/src/main/uk/ac/starlink/topcat/plot2/StackPlotWindow.java

@@ -409,10 +409,12 @@ public void actionPerformed( ActionEvent evt ) {
                                  + " at the bottom of the window" );
         navhelpModel.addChangeListener( new ChangeListener() {
             public void stateChanged( ChangeEvent evt ) {
-                navhelpHolder.removeAll();
                 if ( navhelpModel.isSelected() ) {
                     navhelpHolder.add( navhelpLine );
                 }
+                else {
+                    navhelpHolder.remove( navhelpLine );
+                }
                 navhelpHolder.revalidate();
             }
         } );