fix #430: set unused button invisible

Previously that button was just not layouted in the container widget,
but it was actually usable. Now set the widget to invisible, thereby
removing it from the tab key cycling and from the button click listener.

net.sf.eclipsecs.ui/src/net/sf/eclipsecs/ui/config/CheckConfigurationWorkingSetEditor.java

@@ -356,6 +356,9 @@ private Control createButtonBar(Composite parent) {
       formData.right = new FormAttachment(100);
       mDefaultButton.setLayoutData(formData);
     }
+    else {
+      mDefaultButton.setVisible(false);
+    }
 
     mExportButton = new Button(rightButtons, SWT.PUSH);
     mExportButton.setText(Messages.CheckstylePreferencePage_btnExport);